I just got the 9650 a few days ago and so far the WiFi experience has been great. It was easy to set up here on the home connection and it is noticeably faster. There is still a general lag between when you click something and it snaps into gear and loads it, but other than that it's great! I haven't tried it at any other locations.
As to the operating system question- Barring any new handset releases that happen either before BB6 or at the same time, I would think the 9650 would be the best phone to handle the new operating system because it has the most application memory of any BB out right now (512mb.) So if there are any features that need more room to work with, the 9650 will be your best bet. Other than that I haven't read anything one way or the other about what phones that BB 6 will work on. I'd guess that prettymuch all the newer released handsets will be fine with it, but I don't know that for sure.