Android has seen significant gains in mobile operating system share over the last six months, though Apple's iPhone OS continues to have the highest brand loyalty among its users, according to Monday data from Nielsen.

Research in Motion's BlackBerry platform continues to be the number mobile OS in the U.S., but a poll of BlackBerry users showed that about 42 percent of current owners are weighing a switch to iPhone or Android.

Overall, RIM has 35 percent of the mobile OS market share in the U.S., followed by the iPhone with 28 percent, Windows Mobile with 15 percent, and Android with 13 percent.

Overall, smartphones now make up 25 percent of the U.S. mobile market, up from 23 percent last quarter and 16 percent a year ago. Nielsen predicted that smartphones will overtake feature phones by the end of 2011.

But what OS will people choose? Though Android only has 13 percent of the overall U.S. market share, Nielsen saw a big leap in the number of people choosing Android in the last six months. RIM still came in at number one with 33 percent, but Android overtook Apple to land at number two with 27 percent of recent new subscribers, followed by the iPhone with 23 percent.

While 42 percent of current BlackBerry users will likely stay with the OS when they get their next phone, about 21 percent of BlackBerry users are considering an Android phone, while 29 percent want an iPhone.

Current Android users have a bit more loyalty; 71 percent will stay with Android, while 21 percent want an iPhone and 3 percent want a BlackBerry.

Apple has the most loyal base, with 90 percent of current iPhone users sticking with the OS. About 6 percent of iPhone users said they would try Android, while 2 percent said they would switch to BlackBerry.