I too use T-Mobile.. AT&T Claims "More bars in more places" but T-Mobile and AT&T have a full roam agreement There is no extra charge for roaming (Like there was in the old days) and the two networks are both GSM and there is a full agreement between them so if your AT&T phone works then my T-Mobile works and the other way around. This was not always the case but it is now.
This is also true of most of the other cellular carriers.. If you buy the "National" plan there is no roaming charge anywhere in the United States... Some of them it's anywhere in North America.
Some companies have "emergency" plans.. No minutes (or like 10 minutes a month) for well under 20 bucks even after taxes.
Here is T-Mobile's plan list
http://www.t-mobile.com/shop/plans/
They have plans down to 30 bucks