There are now four different models of 3DS available in the UK, which is confusing by any standards. The New 3DS - out soon - is essentially a new console that's backwards-compatible with the old 3DS' games, but it's a half-step rather than a full generational leap.
I've got all four in front of me. Here's what they look like:
As you can see, the two XL models are identical in size but slightly different in layout, with Start and Select on the right along with that analogue nub. The two smaller 3DSes differ significantly: the smaller New 3DS' screen is bigger than the original 3DS' screen, but still a little smaller than the XL screen.
Here's a quick video comparing the form of all 4 models.
As for what they're like in action: both new 3DSes have slightly nicer screens, with richer colours and less pixellation. I played about ten minutes of Kid Icarus on all of them to get a feel for it, and tried out Smash Bros too for something more recent. Annoyingly, my copy of Monster Hunter 4 Ultimate is digital and currently locked to my old XL whilst I try to decide which New 3DS I transfer all my data to.
Here's the two 3DS XL models running Kid Icarus side by side:
And here's the New 3DS and the New 3DS XL running Kid Icarus side by side:
Here are some more photos of various models next to each other for comparison.
It's clear to me, from playing around with all of them, that the New 3DS is a significant improvement in both screen quality and performance, whichever one you get. It's worth upgrading from an older 3DS model if you can afford it. But, if you're deciding between a New 3DS and a New 3DS XL, it comes down to form factor, screen size and faceplates. Because the 3DS screen tends to be close to your face, I've not found myself missing the bigger XL screen, but when later made-for-New-3DS games like Xenoblade Chronicles come along, I may well wish to admire the souped-up visuals on a larger rectangle.
As for me, for now I think I'm going to stick with the smaller New 3DS as a primary machine. I just like it more, and the faceplates seal the deal.
Time to start that endless data transfer process. Oh, and if you have any questions or requests for further comparison photos/videos, leave them in the comments.