T-Mobile's 3G/4G coverage for an iPhone 5 is better than a year ago but still limited so you are rolling the dice. They just released a carrier update for the iPhone that will give an iPhone 5 access to some of their 4G network. But it's still early days and the usefulness will greatly depend on your location.
T-Mobile also offers a pay-as-you go plan where talk and text are charged as used and data can be activated on a day-to-day basis.
StraightTalk is another option for an unlocked iPhone but is only 3G. $45/month for unlimited talk, text, and data. As usual, "unlimited" means "secret limits" that we can arbitrarily use to kick heavy users off.
The big advantage of T-Mobile is that you can walk into a T-Mobile store and get it set-up. AT&T is still officially restricting the use of non-contract iPhones on their network and walking into a store will be a bust, but they don't actively hunt them down so GoPhone is an unofficial option. You have to buy a GoPhone SIM and cut it down to size but it will work. Unofficial and a bit of a PITA but the option exists. You can buy pre-cut cards on Amazon but getting one in a store is a bit trickier.
I suspect this gives 4G/HSPA+ and not 4G/LTE.