Apart from Apple Inc.'s iPhone 5, the most expected Android smartphones are Samsung Galaxy S2, Motorola Droid Bionic and HTC Thunderbolt 4G. So which is the best Android smartphone that will emerge the winner in the United States?

Samsung and Motorola devices are offering quite a blazing performance thanks to dual-core processing power, but HTC Thunderbolt, despite having single-core processor, which runs on Verizon's 4G network, can manage download speeds up to 12 Mbps, and upload speeds up to 5 Mbps.

Verizon's 4G coverage includes 45 cities, and 60 airports in the U.S. By the end of 2011, Verizon claims that the 4G service will extend to 100 more locations.

However, there still could be a difference or two that would make you choose one over the others. All the three superphones have Google Inc.'s Android OS. HTC Thunderbolt was launched on March 17, but both Samsung Galaxy S 2 and Motorola Droid Bionic are yet to be launched in the U.S.

Outside the U.S., Galaxy S2 was given worldwide release dates from May 2011. The smartphone has sold a whopping three million units outside the U.S., beating the iPhone 4 and upstaging the rumored iPhone 5.

Galaxy S2 will be coming in 4 version in the U.S. -- Function at Verizon, Attain for AT&T, and Within for Sprint Nextel, and a T-Mobile version. Meanwhile, T-Mobile is offering a free Samsung Galaxy S 4G smartphone through web-only 2-day sale with a new 2-year agreement. Motorola Droid Bionic and HTC Thunderbolt 4G will only be featured in Verizon, but Samsung Galaxy S2 will be found in all the four major carriers in the US.

Galaxy S2 is the most successful smartphone in the United Kingdom market and is already considered the best smartphone on the planet. Galaxy S2 is Samsung's flagship phone for 2011 and is the first Samsung phone to be powered by a dual-core processor.

The Galaxy S2 runs Android version 2.3 (Gingerbread) and has an incredible 4.3 inch Super AMOLED Plus touchscreen.

Motorola Droid Bionic is a candy bar-style smartphone with support of modern 4G technology from Verizon LTE network. Motorola Droid Bionic seems to be strong and portable with Android 2.2 Froyo installed in it, which can be easily upgraded to 2.3 Gingerbread anytime.

HTC's Thunderbolt is the first 4G LTE smartphone on the Verizon Wireless network. It is a CDMA variant of the HTC Desire HD. As of spring 2011, this phone is considered the flagship phone for Verizon, going head-to-head with the HTC EVO.

The ThunderBolt is the first phone on Verizon's network that uses a physical SIM card, versus MEID or ESN, for device authentication.

The release dates of Galaxy S2 and Droid Bionic smartphones are making customers wait on their heels. Motorola confirmed via Twitter that its much anticipated Verizon LTE phone will arrive this summer, while Samsung Galaxy S2 is expected to hit the US markets in the third quarter of 2011.

However, the smartphone from Samsung may reach Canada before it will become available in the US market. This comes as Bell Canada has given an official confirmation about the device. Meanwhile, Samsung Galaxy S2's release date is surmised to be July 24 in the U.S.

Here is a comparison of Samsung Galaxy S2, HTC Thunderbolt and Motorola Droid Bionic features.

Operating systems: HTC Thunderbolt runs on Google Inc.’s Android 2.2 Froyo OS. Motorola Droid Bionic will run on Google Android Froyo 2.2, but may be updated to Android Gingerbread 2.3 when Final product comes out. On the other hand, Galaxy S2 runs on Android 2.3 Gingerbread which is the latest smartphone version of Android OS.

Though Froyo isn't quite as refined as Gingerbread, it has true multi-tasking support, Wi-Fi hotspot tethering and Adobe Flash support. But in Gingerbread, the user interface is refined, the soft keyboard and copy/paste features have been improved, and support for near field communication (NFC) has been added.

Camera: The Galaxy S2 has an 8MP single-LED flash camera with 1080p HD video recording at 30 frames per second and 2MP fixed front-facing camera for video chat. HTC Thunderbolt sports a rear-facing 8MP dual-LED flash camera with 720p HD video recording, and a front-facing camera has 1.3MP for video chat. On the other hand, Motorold Droid Bionic has 8MP dual-LED flash camera with 720p HD video record at 30 frames per second and 0.3MP VGA front-facing camera for video chat.

Screen: HTC Thunderbolt sports a 4.3-inch WVGA TFT screen with 800x480 screen resolution. Galaxy S 2 sports a 4.3-inch Super AMOLED screen with 800x480 screen resolution. On the other hand, Droid Bionic comes with upgraded 4.5-inch qHD TFT capacitive touchscreen with 960x540 pixel resolution.

Processor: Galaxy S 2 is powered by a dual-core 1.2 GHz ARM Cortex-A9 Mali-400MP GPU Orion processor with 1GB RAM. Thunderbolt is powered by single-core 1GHz Qualcomm MSM8655 Snapdragon processor with 768MB RAM. The processor supposedly announced in Bionic was 1GHz dual-core ARM Cortex-A9 ULP GeForce GPU Tegra 2 chipset processor, but rumored upgrade is expected to be Texas Instruments OMAP4 1.2 GHz processor with expected RAM to be upgraded from 512MB to 1GB.

Size: The Galaxy S2 has a 125.3 x 66.1 x 8.5 mm and weighs 116 g, while Thunderbolt 4G is 122 x 66 x 13 mm and weighs 164 g. The initial specification of Motorola Droid Bionic shows 125.9 x 66.9 x 13.3 mm and weighs up to 158 g.

Input: The input features found in HTC’s smartphone are: accelerometer sensor for UI auto-rotate, proximity sensor for auto turn-off, HTC Sense 2.0 UI and multi-touch. Galaxy S2 has features like TouchWiz UI version 4.0, multi-touch input, accelerometer sensor for UI auto-rotate, touch-sensitive controls, proximity sensor for auto turn-off, gyroscope sensor and Swype text input. Droid Bionic is expected to include accelerometer sensor for UI auto-rotate, multi-touch input method, proximity sensor for auto turn-off, the latest version of Motorola’s custom user interface, mostly to accommodate the unlocked bootloader option and the user interface is expected to be similar to Motorola Photon 4G.

Storage: Galaxy S 2 boasts of 16/32 GB internal storage and via a microSD card slot it can expand up to 32 GB, while ThunderBolt comes with 8 GB internal memory and via a microSD card slot it can expand up to 32 GB. Motorola Droid Bionic will feature 16GB internal storage and via a microSD card slot it can expand up to 32 GB.

3G: Samsung Galaxy S 2 has HSDPA 21 Mbps and HSUPA 5.76 Mbps support, while HTC’s smartphone has Verizon’s Rev. A up to 3.1 Mbps along with LTE.

WLAN/Bluetooth/ USB: The Wi-Fi 802.11 b/g/n DLNA is common in all the three smartphones but the Galaxy S2 has an added Wi-Fi Direct feature. The Bluetooth version found in Thunderbolt is 2.1 with A2DP and EDR while Galaxy S2 has version 3.0+HS. Bionic uses 2.1 Bluetooth version with A2DP and EDR. Although USB version 2.0 is featured in these smartphones, Galaxy S2 has microUSB (MHL) and USB.

Battery: Galaxy S2 features a battery capacity of Li-Ion 1650 mAh and HTC Thunderbolt has Li-Ion 1400 mAh battery but Motorola Droid Bionic betters all with a Li-Po 1930 mAh battery.

Pricing: The Galaxy S 2 is expected to be priced at $599 to $699 for unlocked 16GB version, while the locked version to be sold on contract from U.S. wireless carriers is expected to be in the $250 to $300+ price range. HTC Thunderbolt is priced at $569.99 on full retail price, while the locked version with 2-year Verizon contract is $249.99. Verizon is offering Thunderbolt for $199.99 with new 2-year contract as online exclusive certified pre-owned device. Motorola Droid Bionic is expected to be priced at $479, but a  $149.99 price tag at Amazon or Verizon store is also speculated.

Other common features: The other common features in all the three smartphones include 3.5 mm jack, loudspeaker, SMS threaded view, MMS, Email, Push Mail, IM, HTML browser, games, Java via third party application, digital compass, A-GPS support, SNS integration, Google Search, Maps, Gmail, YouTube, Google Talk, Adobe Flash 10.1 support, voice-memo-dial-commands and predictive text input.

Other features found in Bionic are HDMI port and active noise cancellation with dedicated mic. Galaxy S2 extra features include stereo FM radio with RDS, optional NFC support and TV-out via MHL A/V link. Also the other features found in the HTC Thunderbolt include video Skype, SNS integration, built-in kickstand, MP3/AAC+/WAV/WMA9 player, DivX/Xvid/MP4/H.263/H.264/WMV9/player and document viewer.

The Galaxy S2 has other features like RSS, Java via third party application, NFC support optional, TV-out via MHL A/V link, MP4/DivX/XviD/WMV/H.264/H.263 player, MP3/WAV/eAAC+/AC3/FLAC player, image and video editor and document editor (Word, Excel, PowerPoint, PDF). Samsung is planning to introduce the Samsung Hub, which will bring users over 2 million books, games from Gameloft and 12 million selections from 7 digital.

Review of Galaxy S2: Reviews of the Galaxy S2 have been positive. Engadget gave the device a 9/10, calling it the best Android smartphone yet and possibly the best smartphone, period. CNET UK gave the device a favorable review of 4.5/5 and described it as one of the slimmest, lightest mobiles we've ever had the privilege to hold.

Conclusion: The specifications of all the three phones look very promising. But Samsung Galaxy S2 looks bright, intelligent and eye-catching, and its ultra thin design may be the most important factor that steals the hearts. Also Galaxy S2 HD 1080p video capture more than just developing Droid Bionic and Thunderbolt supports 720p video recording, and Galaxy S2 sports a 2MP front-facing camera for video chat better than Thunderbolt's 1.3MP and Droid Bionic's 0.3MP VGA camera.