The one drawback as far as I am aware with the Amazon Fire TV is that it does not currently support external hard drives. This means you are somewhat limited as to how many games you can fit on the device. The Ouya supports external hard drives, although in the beginning, it used to have the same problem. If you plan on storing a lot of games at once, or possibly several larger games, then maybe getting an Ouya and an external hard drive or at least decent USB flash drive might be a good option. Currently, the Ouya has more games, but a lot of the big titles overlap and show up on both. If you're enjoying what you have, you're probably not missing a whole lot so far, however, the hard drive issue may be something to consider down the line. I don't know what, if any, progress Amazon has made toward providing external storage support, but Ouya has it now.

tl;dr: If space becomes an issue on Fire TV, Ouya's got ya covered. If not, you're probably good as is.