If you're looking for the most compatible 8-bit, go with the 800XL. It will play just about everything and it's very common. There are a handful of games that have issues with the XL because of the OS change but there's translator carts and disks that fix that problem.

The 400 will work for most cartridge games but it won't play a bunch of them due to it's limited memory (16K when most of the more advanced games needed 48K and occasionally 64K). It also can't support a disk drive because that needs at least 32K IIRC. The 400 can be modded for more memory, but you're better off getting a different system as the mod requires some soldering.

Tempest