Well I'm sure right now you're not going to get a better deal on any other carrier than what you have. I bet you probably have the unlimited package for $80 a month or less.
I briefly contemplated switching to Verizon simply because they will for sure get the Galaxy Nexus, but I figured out that to get CLOSE to what I have now on T-Mobile on Verizon I would be paying almost double.
So some suggestions I have:
1) Do what I did. I ended up buying a Galaxy Nexus straight up. Now that's expensive up front. I paid $720 for the phone, but say you switch to verizon and pay $40 more a month than what you do now, that will add up to $960 extra over the 2 years. Even then you won't have the true unlimited data you have right now, you will get throttled after a while. So you end up basically saving $200.
2) Do option 1, but then since you won't be contractually obligated on T-Mo anymore, you can switch to the unlimited $50 a month plan, which gives you unlimited everything, but your data speed will get throttled after a while after the first 100MB of "4G" speeds. http://prepaid-phones.t-mobile.com/monthly-4g-plans
I highly recommend the Galaxy Nexus, it's a badass phone. Best Android out there by far.
Or if you just want to get a phone from T-Mo, it would be between the Sensation and the Galaxy S2. Both awesome phones.