What is the best iPhone for T-Mobile 3G Network? The 2g or 3g unlocked iPhone?

I plan on buying a iPhone but idk which one is the best for the current network of t mobile. T mobile has a 3g network but i am not sure if the 3g i phone will work with t mobile service. I do not plan on buying internet package service data with t mobile so i am basically only using WIFI. Also btw this iphone is going to be unlocked so i can use it on tmobile.

Thanks