I would go for the 3DS XL. I don't have one, but I'd prefer it over my 3DS.
The 3DS is nice, but the XL has a larger screen, which I also believe is brighter.
And as far as I'm aware (and I hope this is true) it doesn't have the one major pisstake of a design flaw that the 3DS has.
On the 3DS the lower screen is raised out of the actual body slightly by a millimetre or two. This is perfectly fine and there are no problems with this. The problem however arises from the fact that the bottom screen is smaller than the top screen.
When the 3DS is closed the rim of the raised lower-screen touches the upper-screen, some distance in from the outer side of the upper-screen.
Over time this causes the rim to rub and scratch the upper-screen. It starts off by leaving a little white line vertically on the upper-screen, which can be rubbed off. However, without screen protectors (and even these aren't perfect) the line will slowly be rubbed in to a full-blown scratch that can't be removed.
The upper screen is then permanently scratched in a very awkward location. It regularly gets in the way of the images on the screen (and the upper-screen is the focus for most games) and can sometimes distort the 3D effect of that screen.
If the 3DS XL is different and doesn't scratch the upper-screen then I would go for that, solely on the basis of this 3DS design flaw.
I love my 3DS to bits, but whoever designed it seriously screwed up on this part.