The 800XL was a major price reduction from the original 800. In addition to manufacturing and component cost improvements, FCC regulations relaxed a bunch between the original revision and when the 800/600XL came out. So they were able to drop a boatload of RF shielding, just like the VIC-20 and C64 could. The inside of the original 800 looks like a bloody Faraday Cage.
And by that point in the 80s the original 1979 Atari 800 case design was starting to look really 70s. At least that's how it seemed to me as a kid. The 1200XL/800XL case looked so slick. It's up there with the Sinclair QL and some of the Japanese MSX machines in terms of case design, IMHO.
The 800XL is probably one of my favorite 8-bit computers aesthetically, and definitly my favorite amongst ones that I own(C64, CoCo2, Ti-99/4a), thought the Ti-99/4a is close second with that black and metal design, it's just missing some sleekness.
It is all about the nostalgia. I was introduced to this thing called "BASIC" on the 800XL and remember the joy of making the computer beep the theme from Close Encounters of the Third Kind.
For me it was an aesthetic thing. The black modern look with metallic buttons it had was a departure from the beige that dominated computers of the time. Atari had a great design sensibility. The Atari 5200 was another great example of slick design for the time.
the 800xl looked cool, 64k of memory and basically competed head on with c64 until the rise of the next gen Amiga/Atari ST, only to be obliterated by PCs once SVGA came along
To me, the original 800 was the best, but I don't know enough about Atari's eight bit offerings to understand the major differences.