That depends on a few things:
- your budget level
- whether or not you actually care about 3D
- durability
- whether you care about weird feeling of not having a lid close on your handheld
That's...basically it. The 2DS, as far as I know, is super budget friendly, costing around $100 these days if I'm correct, so it's definitely for those who don't really have too much cash to spend but don't want to miss out on the experience of playing the 3DS games. The 2DS, from what I heard, is also more durable than the 3DS since iirc it's originally meant for kids so they can have a system without fear of dropping it or going through a certain level of abuse, etc.
The 3DS itself, on the other hand (speaking on the new 3DS here because the old one is literally ass in comparison), has more color, looks more beautiful to play games with, faster processor which means that your games and everything else would load faster, and if you care about the 3D, it's been fixed in the New 3DS also to a rather beautiful effort. And colorful buttons, so there's that!!
Did this make sense? I can't really give out a recommendation off-hand since I'm not sure what exactly you preferences are...I will say that if you're not a fan of the New 3DS XL's size, then you're probably better off with a 2DS. Otherwise, the New 3DS is significantly better than the older handhelds.