Having played through every game in the series recently, I feel like Dark Dawn's the better game than the original Golden Sun. Not that I'm saying Golden Sun is a bad game, because I really love it, but Dark Dawn's got more content and depth in gameplay (especially when fighting the optional bosses). This is just my opinion, but I feel like it's also got a better atmosphere, more interesting characters, more to explore, and a soundtrack that's just as good as the original. I'd even consider the story to be better, because in the original, you were just chasing the villains the whole time and ended up not resolving anything. The points of no return in Dark Dawn don't really bother me either, because I generally explore places thoroughly and don't like backtracking.
I think, in terms of what the games did for the series, and their quality in comparison to other games that were out at the time, the original is the better game, but for someone who isn't concerned with such things and just wants to play a good game, I think Dark Dawn's the better choice.
I'd recommend TLA over DD any day though.
As for an AT, if Isaac does make it as a playable character, I'd definitely want to see Alex. He'd attack you with snide remarks about your intelligence.