I'm playing Advanced Wars 2 right now (love it) and plan on getting Fire Emblem. Fire Emblem looks like a very different game though than FFTA. It looks more like a FF1 (for the NES) game, only the battles are one-one-one.
Fire Emblem is basically the same as Advance Wars, except with horsies and swords. There is no true character development beyond a hardly noticable stat/leveling system.
Frankly, FFTA and Advance Wars 2 are the two best strategy games you can get on the GBA-SP. Fire Emblem feels like a hackneyed and shallow fan-mod quality game after playing FFTA and AW2.
And good lord don't even think about buying Fire Emblem if you're trying to decide between FFTA and Ogre Battle. Fire Emblem is NOTHING like either of those games.