Which mobile operating system will developers place their bets on in 2011? For most it will depend on a particular platform’s reach, while for others, it’s about what’s “sexy” at the time, experts said.

In an October “State of the Apps Industry Snapshot” survey conducted by mobile advertising firm Millennial Media, “reach” (or the number of consumers in a platform’s audience) was the most important consideration for developers and publishers choosing a platform, while ease of use took fourth place.

Twenty-nine percent of the 336 publisher and developer respondents also indicated they will be creating apps for the Android operating system this year. Windows Phone 7 and the iPad tied for second place, grabbing 20% of the respondents’ interest. RIM took fourth with 12%, while the iPhone trailed in fifth with 8% of developer and publisher interest. However, according to the survey, the iPhone reigned supreme in 2010 with 30% of the respondents developing for it. Twenty-three percent indicated developing for Android last year.

“If you asked me a year ago about Android, I would say it’s looking up,” said Ramon Llamas, senior research analyst for mobile devices and technology trends at IDC. “Now, it’s a question of ‘How big is it going to be?’

“Last year [Android] was kind of a dark horse. We only had a chance to see what Motorola was doing, and now we have a chance to see what HTC and Samsung is doing, and it’s really going to take off.”

The platform’s openness is a great thing for developers, but the downside is its fragmentation, especially for game developers, Llamas added.

Andy Rubin, vice president of engineering at Google and the chief architect behind Android, addressed the platform’s fragmentation at December’s D: Dive Into Mobile conference in San Francisco. Calling it “differentiation” instead (because manufacturers and carriers can tailor Android devices to more or less their liking), Rubin acknowledged that Android is somewhat clunky right now.

“I would characterize Android today as an early adopter for tech enthusiasts…In the future, I think you’ll see a little more attention to detail,” Rubin said, as he hinted toward a smoother user experience coming down the pike.