Had the original 3ds, got a XL now, and will be using the N3DS XL soon.
First of all, my personal reason to upgrade was because the original is too small for my hands and would make them hurt, the XL remedied this problem.
The bigger screensize and low resolution are an unfortunate trade off. Though they only really keep bothering you if you keep comparing, if you still have the old system you'll keep noticing this and won't get used to it.
I had this problem with the spreakers, there is a big difference in volume and clarity of sound between the two because the XL could not fit the same speakers as the original, and this bothered me immensely as long as I had the old one to compare to.
The New 3Ds should offer a solution to both problems, as the normal version wil aready have a slightly larger screen, but not too big to be that aliased. And both will have the better speakers.
I should note that the screen size heightens the immersion of the 3D effect, and i've always played with it on full. A bigger screen also translated to a bigger "sweet spot" for the 3D effect it seemed, though with the new head tracking that won't matter.
I feel sorry the americans can't choose their size on launch, though i'm sure they will come later. If you're not in a hurry I would wait until you tried both systems, no matter what you read it comes down to personal preference. I too love the cripness of the smaller screen, but once you go XL, you never go back.
*edit
Also, higher pixel density screens will always seem a bit brighter/crisper because of the amount of light they emitt devided over a smaller surface. Dunno yet if screen brightness is on the XL is tweaked to compensate this.