Smartphone penetration continues to increase in the U.S., and Google’s Android mobile OS and Apple’s iOS are clearly the market drivers and winners, according to a research update on Nielsenwire.

Forty percent of mobile consumers over 18 in the U.S. own smartphones as opposed to the 60% who own feature phones, Nielsen found in its research for the month of July. With a 40% market share, Android is the most popular mobile operating system (OS) while Apple’s iOS ranked second, with a 28% market share. RIM’s BlackBerry ranked third with a 19% market share, Windows Phone and Windows Mobile fourth and fifth, with 7.1% and 7% shares, respectively.

Looking to what mobile consumers are looking to buy, Nielsen found that 1/3 of those that said they intend to buy a new smartphone in the next year want an iPhone and 1/3 said they want an Android device.

Among ‘early adopters,’ 40% said they wanted an Android device while 32% said they wanted an iOS device. The larger group of ‘late adopters’ are most likely to be unsure and undecided which mobile OS they would want in their next smartphone.