Both have their pros and cons.
>3DS
>Pros
- More portable
- Smaller pixels create sharper looking image
- Sytem looks nicer, has better color options
- Higher quality speakers
>Cons
- Shitty stylus placement.
- Small screen can be hard to see.
- L and R buttons uncomfortable for some.
- Top screen can be scratched by lower screen frame when closed.
>3DS XL
>Pros
- Big screens (a big pro)
- Stylus easily accessible
- Better battery life
- More comfortable to hold.
>Cons
- a bit large to comfortably fit in your pocket
- increased pixel size makes images look a little less sharp, but not awful.
- sound is a little tinny, especially when playing regular DS games
- Top screen is no longer scratched by lower screen frame, but the "open" design of the clamshell now allows dust particles in easily, and they become trapped between the top screen and the frame and will scratch the fuck out of it if you walk around with it in your pocket regularly.
>Con for both
Nintendo seems fine with using shit quality LCD screens on their systems. Both variations suffer from having inconsistent color temperature. It varies from the screens looking a little yellow, to some that look like they are fucking antique parchment. The only systems that are guaranteed not to be affected by this are the original 3DSs made before the price cut. They used a higher quality screen.