Do you have good service where you are with T-Mobile? Staying would be a good idea since their plans are much cheaper than AT&T. The myTouch and the TP2 are both good phones ( I've played with them both).
You could also look into buying a new T-mo phone off-contract (or any other unlocked phone) so that your payment becomes month-to-month, so that you can leave T-Mo whenever you want, so if a really awesome phone comes out on another carrier, you would be free to leave and go get it.
Personally, I considered switching to AT&T, but since I've been with T-Mobile for a while, I get the unlimited loyalty plan. So for $84.99 a month, I get unlimited minutes, unlimited internet & email, and unlimited texts. I'm happy with T-Mobile, with no plans of switching anytime soon.