uhm..... the only major difference than that is online, but I'd go for fire emblem... I'm actually playing it right now xD
I guess it all comes down to what you like.
Radiant Dawn has some pretty sweet cinamatics, or trailers, in the gameplay, but graphics are a bit better, but not to much, during fighting.... anddd..... uhmmm I haven't played Battlion Wars, so I can't say much about it.
If you liked the storyline from POR then I'd get it for sure.... but I never finished POR, I rented it. If you could, rent em first, then that would be good, cause then you can just pick for yourself