3DS is quickly becoming THE handheld for fighting games fans and i think it's really great.
i would recommend everyone to get both SSFIV3D and DoA: Dimensions, they are both great games and they sorta complement each other. SSF has great multiplayer mode, while DoA shines in single player.
SSFIV is a really great port but it still is a downgraded port so if you don't have the need for a portable version of the game and you already own it on other platforms (or if you could get it on other platforms) i think you should get DoA. otherwise is though decision.
As for the other fighting games on 3DS:
BlazBlue: Continuum Shift II. It's fun how everybody seem to forget this game. BTW it's a bad port of a good game. the fighting system is superb, and there are tons of contents, but the framerate is all over the place and there is no online play. Get the Extend version on PS3, Xbox360 or Vita instead.
Tekken 3D: Prime Edition. this one is horrid. this is a port of tekken 6 stripped to the core. no arcade mode, no story mode. there are only two singleplayer modes (quick battle and survival) and the game doesn't do anything to introduce you to the characters, to the fighting system (we shouldn't forget that this is the first time in 10 years that tekken comes to a nintendo platform so i think that lots of nintendo fans aren't familiar with the franchise) or to keep your interest. the online play is a mess. the game forces you to play in 2d mode when online and there is a tremendous lag which makes the game unplayable. So let's recap: no single player, no multiplayer, so what's this game all about? oh yeah namco threw in the movie tekken blood vengeance, which is enjoyable even if it isn't all that great, but the video player is horrid. it is impossible to stop and resume the movie if you exit to play the game or do something else with the 3ds, navigation is a mess and even skipping to different chapters is complicated. there is absolutely no reason to get this game. sure there are 40 fighters in there. sure it has the usual tekken gameplay. shure it is locked on 60FPS, but in the end you can't squeeze a single drop of fun fromthis one.