I love the Atari 8-bit line and there's nothing at all wrong with picking up one of those. But I also want to put in a good word (along with a few caveats) for the 5200.
The 5200 is a great system for the collector/enthusiast. It looks great, has great looking carts, and it's not that hard to put together a complete collection of its 69 original vintage games. In fact, the 5200 carts are usually cheaper that their 8-bit counterparts (Bounty Bob Strikes Back, however, being a BIG exception!) While all it's games have indeed been ported back to the 8-bit line, remember that getting them to play them isn't as easy as slapping in a cart... you'll need to either put files on a disk, or set up a SIO2PC solution, or put them on a flash cart, etc.
The 5200 puts out a strong RF signal that looks pretty damn good to me (certainly better than my other Atari systems.) While there's been a lot of fuss over the whole 2-port vs 4-port issue, either one is fine. The 4-port has the advantage of being able to play three carts that the 2-port won't (Pitfall, Mountain King, and Krazy Shootout.) It can't use the 2600 adapter, however, and some people are uncomfortable with the power going to it's switchbox. Myself, I've never had a problem with that.
The biggest issue that most 5200 owners face is the controllers. There are folks like Mirage who like the original controllers, but plenty of other folks who don't, myself included. Everyone agrees that they aren't very reliable as originally sold. These issues can be fixed by either the Best gold kits, or by the much cheaper solution of gluing bits of aluminum foil to the carbon contacts (lots of threads in the 5200 forum on how to do this.) But neither solution will fix the issues of the non-centering sticks, nor the pain many of us get from using side-mounted fire buttons. To avoid these issues, I highly recommend getting either a Wico Command Controller (analog) or a Coin Controls Competition Pro (digital) to complement the 5200 sticks (which are still used for the keypad and Start/Pause/Reset buttons.)
On the other hand, if you'd rather not deal with the 5200 controller issues, aren't intimidated by learning how to get software onto the 8-bit computers, would prefer a vaster library of software, and don't especially care for the aesthetics and collectibility of the 5200 and its carts , a 800XL is an excellent machine. Really it boils down to a choice between one of the very best 8-bit computers of its day, or a great game console with some controller issues that can be overcome. Either choice will give you a whole lot of fun!
Edited by sdamon, Thu May 17, 2012 10:24 AM.