Samsung Galaxy S2 continues its strong march in the smartphone race, giving stiff competition to Apple's iPhone 4 as well as the rumored upcoming iPhone 5.

The device, which was recently launched in the United States, sold a whopping 10 million units since its launch in late April 2011, not including sales from the U.S., the largest market for smartphones.

On the other hand, Apple sold 20 million iPhones in the third quarter. Though the Galaxy S2 sales lag iPhone sales, it implies that Samsung Galaxy S2 is the toughest rival Apple would be facing for its iPhone 5, which is expected to be launched in October.

Apple's iPhone 5 is expected to give all the smartphones a run for their money due to its advanced technical specifications and a strong ecosystem consisting of iOS 5, iCloud and over 420,000 apps.

But, it would be difficult for iPhone 5 to surpass the Galaxy S2, which is currently the world's thinnest dual-core smartphone.

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, offering the richest video content on a smartphone. The phone also has the front-facing 2-megapixel camera for video chat.

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

Why Galaxy S2 could be a strong contender against the iPhone 5:

First, Galaxy S2 scores over iPhone in the processor game. A smartphone with a good processing speed is always an added feature as it improves the overall performance of the device. The Sprint and AT&T version of Galaxy S2 is equipped with dual-core 1.2 GHz ARM Cortex-A9 Mali-400MP GPU Orion chipset, while the T-Mobile version is expected to be powered by a 1.5 GHz dual core Qualcomm APQ8060 processor. This is almost equal to the processing power of a desktop machine.

On the other hand, the iPhone 5 is expected to come with the iPad 2's A5 processor but it is doubtful if it would exceed the 1.2 GHz barrier. With improved versions of Galaxy S2 about to feature 1.5 GHz processor with 3D graphics support, iPhone 5 needs more to match up to Samsung's flagship phone.

Meanwhile, Galaxy S2 sports Super AMOLED Plus display, which is considered to be the best available technology for mobile displays. The iPhone 5 is said to be an improved version of iPhone 4, but still will not be able to match Super AMOLED Plus, which delivers superior color, enhanced readability, slimmer form and better battery consumption than the previous generation. The Real-Stripe panel technology brings 30 percent more pixels per inch for a brighter, clearer and richer viewing experience compared with the first-generation AMOLED.

The Apple iPhone 5 is expected to feature a 3.7-inch or 4-inch display according to different case designs. At a minimum of 4.3 inches, Galaxy S2's display dwarfs the upcoming iPhone 5. With major smartphone manufacturers opting for 4.3 inches as the ideal screen for their phones, it will be interesting to see what comes out on the new iPhone 5.

Now comes Flash, which enriches Web experience and is considered as a big advantage an Android systems holds against iPhone despite the industry shift to HTML 5.

The Galaxy S2 supports full HD video playback inside Adobe Flash player and there is no other device at present which allows full HD video playback even inside Adobe Flash player. But Apple criticizes Flash saying it is a big security risk, not to mention it crashes lots and kills your battery life. Rather than using Flash, Apple has adopted HTML5, CSS and JavaScript - all open standards.

Recently, Adobe said Flash will come to iPad, iPhone, iPod Touch, not directly supported but just that the new app tools will be able to export Flash content as an HTML5-supporting format.

On the connectivity front, Galaxy S2 already supports 3G data speeds up to 21 Mbps (HSPA + networks), but with the Galaxy S2 LTE version poised to feature HSPA up to 42 Mbps, the iPhone 5 doesn't stand a chance in terms of speed. In addition, the Galaxy S2 smartphone will get the 4G treatment, offering data speeds up to four times faster than what is expected in Apple's forthcoming iPhone 5.

Many reports and rumors show that 4G capable iPhone 5 is on the way, but in fact it may be 4G supported through AT&T network and most probably not the Verizon's LTE.

iPhone 5 due this fall will support HSPA+ technology, a development that will allow the device to offer faster data speeds on GSM networks such as AT&T, Hudson Square analyst Todd Rethemeir wrote in a note to clients..

Last but not the least, Samsung Galaxy S2 supports the Near Field Communication (NFC) chip that allows for simplified transactions, data exchange and connections with a touch. As of now, NFC is just a rumor for iPhone 5 and most probably Apple won't be embracing the technology, since NFC has no industry standards.

Globally, 100 million people use mobile payment outside the U.S., but only 3.5 million use the technology in the U.S. It is said that Apple has disclosed in some of the meetings that NFC won't be on the feature list of the next iPhone.