They both are pretty similar to me, although they both have different strengths. American has been around longer so therefore it offers deeper alumni base which is a big deal. However, it is too expensive. George Mason is not that old so they have fewer alumni base, but they do have an upward trend in rankings and they are moving up pretty fast. Infact if you look at their acceptance numbers, they seem to be a whore for LSAT scores lol (even more so than other law schools). Plus they are extremly cheap, and even cheaper in your second year when you become an instate student (although I dont know how hard/easy it is to turn into instate student). So based on that I would choose GMU seems to be clearly better choice in my opinion. If finance is not an issue, I think you are basically spliting hair since their placement in DC is not that far apart. Maybe then I would consider it to be a difficult decision.
But again this is advice from an OL who has done some reasearch. Maybe current Law students can shed a better light on this subject than me. Regardless, good luck with your decision.