I recommend the regular n3DS, it's more comfortable to use in any position as it's light enough to hold one handed with ease, even for me with huge hands.
The screen looks much sharper than the XL due to the XL having an oversized screen for the resolution.
And as a plus, you get pretty faceplates
I own both the original 3DS, 3DS XL and the n3DS, and I think n3DS got the size just right. It still looks about as sharp as the original 3DS screen, but doesn't feel tiny when I'm playing on it, like the original one did after having used a XL once.