Samsung is launching its flagship Galaxy S2 smartphone in the U.S. to compete with Apple's iPhone 4, setting a stage for an ultimate smartphone battle.

Galaxy S2 comes to the U.S. after selling a record 5 million units in 85 days in Europe and Asia, and is the best-selling Samsung phone ever.

In the U.S., Galaxy S2 would be carried by AT&T, T-Mobile and Sprint, whose version would be called as Samsung Epic 4G Touch.

AT&T's version is the thinnest of the three at 8.89 millimeters, usurping the Sprint version's 9.65 millimeters. AT&T's version would have a 4.3-inch screen, while the Sprint and T-Mobile versions are expected to sport a 4.5-inch display.

On the connectivity front, the smartphone will use HSPA+ network on AT&T and T-Mobile and WiMAX on Sprint.

On Sprint, Epic 4G Touch will cost $199.99 (excluding taxes) with a two-year contract. AT&T version is also expected to be on the same lines in the range of $250 to $300, while T-Mobile hasn't announced pricing or availability details.

Galaxy S2, which runs on Android 2.3 Gingerbread OS and sports Samsung's TouchWiz UI, comes with an 8-megapixel camera with flash and can record HD video in 1080p. The phone also has the front-facing 2-megapixel camera for video chat.

The device is powered by a 1.2 GHz dual core ARM Cortex-A9 processor running Samsung's Exynos 4210 System on a chip (SoC).

The Galaxy S2 has a Voice Talk feature allowing users to give their Galaxy S2 device voice commands, including voice dial, text message, navigation, music, Web browsing and search. When docked into the car dock, the Galaxy S2 automatically defaults into Voice Talk mode.

Galaxy S2 also supports popular video conferencing services like Skype, Fring, Qik and Tango and has a Super AMOLED Plus touchscreen display.

The device features six-axis motion sensing using an accelerometer and gyroscope that support advanced touch screen gestures, including motion to zoom in on images, silence a ringing phone, and even move widgets on the menu screens.

Galaxy S2 is expected to come to Sprint's network on Sept. 16 and would be launched on AT&T from Sept. 18.

Rest of the specifications is same for the three versions. Let's take a look at how Galaxy S2 weighs in against the iPhone 4:

Operating system: Samsung Galaxy S2 features the Android OS version 2.3 (Gingerbread) and iPhone 4 runs on iOS 4.

Processor: Samsung Galaxy S2 comes with dual-core 1.2GHz ARM Cortex-A9 Mali-400MP GPU Exynos processor with 1GB RAM. Apple packs an A4 processor and 512 MB of DRAM in iPhone 4.

Screen: Galaxy S2, in general, sports a 4.3-inch Super AMOLED screen with 800x480 screen resolution. iPhone screen which comes in at 3.5 inches with 960x460 pixel resolution.

Size: Generally, Samsung Galaxy S2 has a dimension of 125.3 x 66.1 x 8.5 mm and weighs 116 g. iPhone 4 has a dimension of 115 x 58 x 9mm and weighs in at a slightly heavier 137 grams.

Camera: Samsung's Galaxy S2 has an 8-megapixel camera with auto-focus, LED flash, geo-tagging, touch focus, face and smile detection and image stabilization. Galaxy S2 captures HD video at 1080p at 30 fps.

iPhone 4 has a 5MP camera with and LED flash, capable of recording HD videos at 30 fps, while having a VGA camera in front for video calls.

Storage: Galaxy S2 boasts 16GB internal storage and can be expandable up to 32GB via a microSD card. iPhone 4 comes with 16GB and 32GB models.

Input features: Galaxy S2 has features like Gorilla Glass display, TouchWiz UI v4.0.  iPhone 4 has a multi-touch display, push buttons and dual microphone.

WLAN/Bluetooth: Galaxy S2 has Wi-Fi 802.11 b/g/n DLNA support and an added Wi-Fi Direct feature. Samsung's phone uses advanced 3.0+HS version Bluetooth with A2DP and EDR.

On the other hand, iPhone 4 also has Wi-Fi 802.11 b/g/n and supports Bluetooth 2.1 and EDR.

Battery: Galaxy S2 features a Li-Ion 1650 mAh battery, while iPhone 4 has 1420 mAh battery.