I kept my 6610 for a very long time, keypad was excellent one of the best I used. Its just a great phone for calling and texting.
What sort of contract are you looking at...i.e. monthly minutes/txts ? If you plan to buy direct from the network providers it will have to be an 18mnth contract for best value. Check out T-mobile flext.
Personally I don't wish to be locked into an 18mnth contract (nice to have a new phone after a year!) so usually buy an 12 month deal through
www.e2save.com. Its a redemption scheme and works out much cheaper in the long run.