Buying off contract?
Got about $500?
1520.3... It's what I upgraded to from a T-Mo branded 925. Future proof and one of the 3 most powerful Windows Phones out right now (930, but no Glance and poor T-Mo compatibility for signal or One (M8), but no date for arrival on T-Mo - these are the other two).
If you need the payment plan option, wait a bit and see... There are rumors floating about. I don't think 635 is enough of an upgrade to make you happy if something big comes along inside 6 months.
You definitely want current gen chipset to get all goodies coming in Denim and beyond. That's what upgrades are for.