Well in my experience I tested both of them before buying. In the end I went de N3DSXL way and here is why.
The N3DS:
Pros:
-the small size makes it lighter and more portable. It fits in almost any pocket.
- faceplates are great if you're into that.
Cons
- the small size (yes it is also a con) makes it feel like a toy that will break on you or at the first drop. (I know it will probably survive and last long but just my thoughts). Additionally to the size your hands will cramp in long play sessions.
- the screen size, at first I thought the Xl screen was just a gimmick but in ported games like OOT the size matters, believe me your eyes will thank you.
As for the N3DSXL
you have less personalization options but you get better battery, screens, your eyes will be less sore after long plays and your hands will cramp less as long as you play in a good position.