Moved to a more appropriate location.
The 8530 is CDMA only and will not work on T-Mobile, if that is the carrier you will use. The 9650 is CDMA and GSM and will work on T-Mo, and I don't think it will work on T-MO's US 3G network - doesn't have the right frequencies.
Speed depends on too many considerations outside your control so it is difficult to say (the connection, processor, memory, distance to the tower, tower congestion, etc.). However, there is generally a bit faster download when tethering.
As for OS 6, don't count on it working that well on the 9650. It is designed for the newer devices such as the 9800 with more memory and a faster processor. If there is a version for the 9650, I doubt if anyone can tell how well it will work, what changes are made or when it will come out. Everything is still rumor as far as I know - nothing official is out there.