For me, Super Mario 3D Land was one of the very best uses of 3D so if that is a major reason for buying the system, I’d lean toward 3DS XL.
That said, there aren’t a ton of games where the 3D is massive although I really have enjoyed all the remakes from the 8 and 16-bit days that have been released to take advantage of 3D. Parallax scrolling with 3D is superb!
So it’s really up to you? How much will that extra $50 grind your gears? You can do without it and you won’t really know what you’re missing specifically, but it can be really cool.