iPhone loyalist now ready to switch to sprint or t-mobile...advice?

iPhone loyalist since 2007 - 2G, 3G, 3GS, 4. I'm over ATT. Horrible service, and a fortune each month. I'm strongly considering Sprint or T-Mobile. Being a google apps enthusiast, im not TERRIBLY devastated at switching to droid, but I want a killer phone. Thoughts? Evo seems to be topping my interest charts.

EDIT: I am presently with T-Mobile - it blows horribly on the iphone; edge network sucks and i know im missing text messages, no mms, etc

EDIT 2: I am WAY open to sticking with TMO on the iphone, but i need better than edge, and a stronger mastery over cydia and jailbroken iphones - where can i learn the basics?

EDIT 3: Everyone is super helpful! In the end - i want to stay with apple - just not pay 130/month - sprint has the same service for 70...

EDIT 4: I'm in Boston