Looked it up a bit ..
"T-Mobile USA, Inc., today announced an industry first — a truly Unlimited Nationwide 4G Data plan. Featuring no data caps, speed limits or bill shock, as well as fast, dependable nationwide 4G coverage, T-Mobile’s new Unlimited Nationwide 4G Data plan offers the ultimate worry-free experience. The new plan is designed to satisfy both data-hungry customers who want to experience all their smartphones are capable of and those wanting the peace of mind of never having to keep track of their data usage.
The Unlimited Nationwide 4G Data plan will cost $20 per month when added to a Value voice and text plan or $30 per month when added to a Classic voice and text plan. For example, a single line Value plan with unlimited talk and text combined with unlimited nationwide 4G data will cost $69.99 or a single line Classic plan with unlimited talk, unlimited text and unlimited nationwide 4G data will cost $89.99."
This why I should go T Mobile instead of AT&T?