The 9850 is a CDMA device. As far as I know only T-Mobile offers UMA in it's phones so that means you would want the GSM 9860. That's assuming T-Mobile's planning to carry it. I haven't heard one way or the other if they are. They just released the 9900 w/o UMA but will add it in a software update. Their version of the 9860 would probably be handled the same way.
