Samsung passed Motorola three years ago to become the biggest mobile phone maker (feature+smart phones) in the US, so their operating system choices carry a lot of weight.

They are also listed as one of the five Windows Phone 7 Series launch partners. But at IFA this week, and in earlier statements by its CTO, Samsung seems lukewarm on Windows Phone 7’s prospects, while at the same time lining up its game plan behind the exploding Android platform.

Samsung isn’t just any Microsoft (MSFT) launch partner. It is also the manufacturer of the prototype Windows 7 phones going out to developers and journalists for pre launch.  They’ve said that this device won’t be among the final Windows 7 products, however.

But is Samsung even planning to be in the Windows 7 universe for the long haul?  Engadget spoke with Samsung’s CSO Omar Khan who (see 12:45) wouldn’t commit to having a Windows Phone 7 Series at launch.  He really had next to nothing to say about Windows phones in general.  Khan had plenty to talk about in the Android world, however and spent most of the interview talking about upcoming Android products.

But that’s just the tip of the iceberg this week.  At IFA, YH Lee, head of marketing at Samsung Mobile, told Reuters, “We are prioritizing our Android platform. Android is very open and flexible, and there is a consumer demand for it.”

What about Windows Mobile?

Samsung will introduce a smartphone running Microsoft’s new Windows version later this year. “There is still some professional, specialized demand there,” she said.

Ouch! (as a sidenote, she also that Samsung wasn’t going to build any more Symbian phones either).

The Android strategy has been working so far for the S. Korean conglomerate.  Samsung sold a million Galaxy S phones in 19 days before it was even brought to the US, the largest smartphone market on earth.   Once here, it sold over a million devices in a month in a half on AT&T and T-Mobile.  It launches/launched on Verizon/Sprint this month.

Other manufacturers are also putting more weight behind the Android line.  Another Windows launch partner, Dell (DELL) was rumored to be pulling out of the launch, though Dell say they are still in the Windows game, though it isn’t certain at what capacity they will be playing.

What about Asus, LG and HTC, the other three launch partners for Windows Phone 7 Series?   LG and HTC have hearty Android products and even bigger plans.  Windows clearly isn’t the first priority for these two companies.  Even ASUS, who comes from the PC land and switched its entire netbook line to Windows a few years ago, is now building Android mobile devices like the Garmin phone.

Windows 7 Phone Series is scheduled to be launched for the holidays and Microsoft plans to put hundreds of millions of dollars into marketing the OS.  Google (GOOG) however, isn’t standing still.  They have the third major version of their Android OS, called Gingerbread, which is expected by that timeframe as well.

For these hardware manufacturers who have a choice in which OS to put on their devices, it all comes down to what consumers want.   With Android popularity skyrocketing, it is risky to spend a lot of time and money developing for Windows Phone 7, even with a monster marketing budget like Microsoft’s. Microsoft put a lot of money into marketing Kin and how well did that do for Sharp, its manufacturing partner?

The battle begins this holiday season and expect to be bombarded with advertising from both Microsoft and Google.  As consumers, we’ll have more  mobile phone OS choices this year than we’ll ever have again.