I used to have t-mobile as well. I switched over to verizon about a year ago. The quality is about the same, the only difference is you get MUCH better signal. I get more bars everywhere. In my opinion, If you already get good service where you use your phone than just stick with a gsm carrier...
I say you should just build one yourself. Its rather easy and will save you some money. You can get the parts from all over the place; ebay, websites, stores, etc.