Well, like CaptRory said the games aren't anything alike, so it's difficult to tell you which one to get, but I'll try to help out.

If you have a lot of time to kill, get FFTA because that easily took more than 100 hours of my life away.

Get FF X-2, if you want a fast paced game that's more like other FF's.

I wouldn't know what to pick either, so good luck making a decision
I"d hate to be in your position right now