there isnt even a release date yet
my advice is to get an XL, and once the new 3ds comes out, trade in the XL and buy the new one with store credit
Trade ins are bull, specially when it comes to Gamestop, Rumors are its coming around December or March 2015.
Why December? Christmas, and that makes it so all of a sudden there is a most wanted christmas gift.
Why March? Thats when the first 3DS came out.
Here is a size comparison on the N3DS and the N3DS LL

The LL is based on the Gamecube Color Scheme.