I just switched from AT&T to T-Mobile earlier this year. Even though we don't have their LTE in our area yet their HSPA+ speeds are excellent and I'm surprised they have much more consistent signal in the area around home.
Hope T-Mobile works well for you, I'd like to hear what you think of their LTE, and I hope you love your new phone too. Have fun discovering the joy of widgets and all the rest!
:-D