Galaxy Nexus GSM - AT&T or T - Mobile

I currently have a GSM Galaxy Nexus with a T- Mobile Prepaid Data plan ($30 - 5GB). I love the phone and service but I do live in an area were if I leave the city my signal drops to EDGE or nothing at all. I have been thinking about trying AT&T instead to help with reception issues.

Will switching to an AT&T plan help out with roaming? I check with T-Mobile and my current plan does not have roaming included.

Also, building penetration is extremely poor. Outside I can get between 5-6 mbps down and up. Going inside drops performance down to 1.5-2 mbps down and up. Is this a T-Mobile specific issue? Will switching to AT&T help with building penetration?