Really, to guarantee WM7 you would need to wait.
I would ask you two questions.
1. Do you fall into the camp that requires a physical keyboard? (I do)
2. Do you object to flashing a new ROM on your phone? (Potentially voiding warranty but you can revert so it has never been an issue for me, and I have never bricked my phone despite years of flashing new ROMs)
With Windows mobile phones, there are so many resources over at XDA developers (although this is for HTC made phones).
Do you intend to buy outright or get it on a contract?.
I have a Touch Pro 2 on Vodafone - it came with 6.1 and they have just issued the upgrade to 6.5. Having said that I have been running 6.5 for months (from the aforementioend source).
The latest and greatest to look for is the HTC Leo, which will come out as the HTC HD2. Should be out this month or next. Bit too big for me and no keyboard, but is the next step up in performance, and I would have thought that WM7 will be available for it even if not officially.
With the way things go, I would not expect to see WM7 on phones officially for over a year, by then there will be a lot of new phones out with quite a step up in performance.
I love my TP2, and am using it with SPB Mobile Shell3.0. Key other programs that I use are PhoneAlarm and TomTom.