Apple

The iPhone 4S was the top selling smartphone at all three of its U.S. carriers last month, but Samsung kept nipping at Apple's heels, according to investment firm Canaccord Genuity.

All three models of the iPhones 4S (16GB, 32GB, and 64GB) grabbed strong sales and a higher market share in the U.S. last month, with a little help from Black Friday and Cyber Monday. But so did Samsung's Galaxy S II.

Samsung's latest smartphone was the top seller at T-Mobile and the second best-selling smartphone at both AT&T and Sprint. The Galaxy S II is offered by all the U.S. carriers except Verizon Wireless.

Sales of the iPhone 4S weren't as strong at Verizon as they were at AT&T and Sprint in November, says analyst Michael Walkley, citing two reasons why.

Verizon kept pushing its new LTE smartphone plan offering two times the amount of data under the same price structure. And Verizon customers who bought the iPhone 4 last year were stuck in the middle of their two-year contracts. Still, the new iPhone managed to beat the Galaxy S II to the top of Verizon's charts.

But Samsung's upcoming Nexus phone is expected to make a further dent in the market. Motorola's Droid RAZR was Verizon's top selling LTE Android smartphone over the past two weeks. But Walkley expects the Nexus to bite into Motorola's sales and present a challenge to Apple.

"Through year-end, we expect strength for Apple and Samsung will continue, as we believe Samsung's sales at Verizon will ramp into year-end with the upcoming launch of the Nexus Prime, the first Ice Cream Sandwich Android smartphone," the report said.

The battle for market share between Apple and Samsung has also hurt the other players, such as HTC. The launch of the iPhone 4S and Samsung Galaxy S II took a bite out of HTC's share across all four U.S. carriers in November.

Research in Motion was also affected by competition last month, not only from Apple and Samsung but from other vendors.

Following strong sales for its Bold 9900 handset, RIM saw a decline in revenue in November due to lower demand for its high-end phones and the popularity of Nokia feature phones and inexpensive Android phones in emerging markets.

Beyond the U.S., the iPhone 4S has seen robust demand around the world and is slated to hit several major new markets this month, including China, Brazil, and Russia. As a result, Canaccord Genuity has upped its fourth-quarter sales estimate for the iPhone to 30.5 million from 29 million previously.

"We believe Apple will post strong market share gains during Q4/11," the report noted. "With the iPhone 4S launching soon in Brazil, we anticipate strong sales trends in this leading emerging market and momentum for iPhone share gains to continue exiting 2011."

Despite Apple's strong gains, however, Samsung will continue to chomp off a bigger chunk of the smartphone market, forecasts Canaccord Genuity. For 2011 as a whole, the firm sees a 19.5 percent slice of the smartphone market for Samsung and 18 percent for Apple. Samsung's share will continue to rise next year and finish 2012 still ahead of Apple.