The current T-Mobile Simple Choice plan for 1 GB of 4G data comes out to $50. The $60 option includes 3 GB of 4G data. Both are throttled unlimited data plans with no overage fees.
The current AT&T Mobile Share plan with 2 GB of data (capped with overage fees) runs $80, with a discounted rate of $65 if you bring your own device or go on their Next upgrade plan.
All of these options include unlimited talk and text.
As mentioned elsewhere, T-Mobile has a less extensive network, so you'll likely find more coverage gaps when you travel outside of metro areas. Also, T-Mobile's signals also have less penetration into large buildings. This issue will be somewhat negated when T-Mobile's wi-fi calling comes to the iPhone with iOS 8.
The T-Mobile phones that you buy from Apple are all unlocked, so you when you go overseas, you can use an international SIM card. The iPhones that you buy from T-Mobile stores require that you sign up for 40 days of service and pay off the phone in full before they'll unlock it.
I believe that AT&T will unlock the phone as long it's paid off. If you pay for an AT&T iPhone in full at an Apple store and request the device-only option, it will come unlocked out of the box.