Batman AA and AC are VERY different than GTA, Assassin's Creed or Saint's Row.
GTA is an open world game that usually takes place out doors and revolves around stealing vehicles and (mediocre) gun combat.
Saints Row is a direct competitor to GTA but amped up to ludicrous proportions with over the top action set pieces.
Assassin's Creed, shares SOME similarities but it has a terrible, convoluted story and also primarily takes place out doors. The elements it shares with B:AA & B:AC are all better done in the Batman games.
If anything Batman is a marriage of a Metroid game, where you are acquiring new gadgets that allow you accesses new areas, and a Splinter Cell game with it's stealth. Biggest difference there is that Batman's stealth is actually FUN.
But maybe the best reason to play the Batman games over those others is that B:AA and B:AC have frickin' Batman.