I've played the demo on the XL and have the real game for a Japanese original/vanilla 3DS and here are my thoughts:
Vanilla- It's actually pretty good. I was surprised that it looked so nice. 3DS games always look better ingame than in pictures, but it's just stunning. The characters are almost adorably small, but they're easy to see most of the time. I've only ever had problems when people run to the opposite end of the stage on large ones and when playing as DHD and the camera pans out to follow his B attack. Still, it looks and feels great.
XL- I was expecting a massive difference, but to my surprise, there wasn't one. Yeah, it looks a bit nicer and I like the size, but it wasn't noticeably nicer. Control-wise, it felt clumsily large in my hands after I had gotten used to the 3DS. True, I'll probably readjust when the English one comes out, but it's saying something when the vanilla 3DS controls just as well as the XL.
All in all, while I like the larger XL and am looking forward to playing Smash with it, I don't think that it's necessary. If you want to wait, get the New 3DS XL when it comes out internationally and rest easy knowing that the game will play just fine on your vanilla 3DS. If you don't want to wait and want to play it big, buy the special edition Smash 3DS XL since you'll get good usage out of it anyway, and if you want the New 3DS XL down the road, you can always trade in your old one for some money off on the new one.0