According to a survey by NPD Group, during the fourth quarter of 2011, the iPhone accounted for 43 percent of total smartphone sales in the U.S. However, in terms of OS, Android smartphones grabbed the top spot by accounting for 48 percent of total sales.

Together, iPhone and Android devices accounted for over 90 percent of smartphone sales. It means other OSes like BlackBerry, Symbian, Windows Mobile and Bada got to share what's remaining on the plate - 9 percent.

iPhone 4S, iPhone 4 and iPhone 3GS were the top three best selling smartphones, followed by Samsung Galaxy SII and Galaxy S 4G.

Wondering what made iPhone 4S the best selling smartphone? That's a no-brainer. Of course, it's because of Siri, the intelligent voice assistant, a feature that's exclusive to iPhone 4S.

No wonder, NPD said iPhone 4S outsold iPhone 4 by 75 percent.

Siri has endeared itself to iPhone fans ever since Apple's latest smartphone was launched. It can do almost anything the user asks it to - send an email, call a person, search for an address, take a note, open a Web site and more.

Other features like a faster processor, improved camera also helped.

iPhone 4 grabbed No.2 spot, probably because it's being sold at a discounted price of $99 (8GB model).

And, though iPhone 3GS debuted in 2009, it has managed to grab the No.3 spot because it's currently being given away for free with a 2-year AT&T contract.

According to UBS analyst Maynard Um, Apple and Samsung will capture more than half of the smartphone market. So, if RIM, Nokia or even HTC complain, we won't be surprised.

However, Apple better watch out - the survey also revealed that first-time smartphone buyers are leaning towards Android device than iPhone.

Android smartphones were the No.1 choice of 57 percent of first-time smartphone buyers compared to iPhone, which was the first choice of 34 percent first-time buyers.

This is surprising because Android OS has often been criticized for not being as user-friendly as iOS. Android has been criticized for offering a more complex user experience than its competitors, but the company's wide carrier support and large app selection is appealing to new smartphone customers, NPD's executive director Ross Rubin said. Android's support of LTE at Verizon has also made it the exclusive choice for customers who want to take advantage of that carrier's fastest network.

However, NPD's survey needs to taken with a pinch of salt because a recent comScore report had put Android OS/devices way ahead of iOS/iPhone (47.3 percent versus 29.6 percent).

And then, there's Nielsen, who recently released a data which suggests that Android's market share among new purchasers, plummeted from 61.6 percent in October to 46.9 percent in December, while iPhone's market share rose from 25.1 percent to 44.5 percent.

Why the discrepancy? The reason is because they all have used different methodologies to arrive at their conclusions.