I would buy an update that is maybe slightly bigger than the original 3DS but not as big as the XL. The XL looks nice, and I absolutely love the start/select/home buttons, but it's just too big for a handheld for me, it's got to fit in my pocket.
Things I would want are the start/select/home buttons from the XL, better battery life, bigger screens (better resolution would be nice), much better cameras, a less blocky design, and fix the screen scratching issue. I don't care at all about it, but they might as well throw a second analog on there to make people happy. Oh, and put the stylus on the right side so it's easy to get to.