R_Leo_1

Wow, that's a tough one.  They both play many of the same great games (although sadly they can't use the same carts.)  I love the look of both machines... which appeals more to you?  Do you want something that's potentially expandable to have more RAM and external storage?  Then it's easily the 400.  Or would you prefer to collect a small but excellent library of large, fun-to-plug-in carts?  Then it's the 5200.  

A big consideration is the controls.  The 400 uses standard 2600 joysticks and paddles... simplicity itself.  The standard 5200 controllers are famously unreliable, non-centering, and many find them uncomfortable.  Replacements exist but they're usually quite expensive.  

If this is your first Atari 8-bit machine based on the 400/800 architecture, I'd probably lean a bit more towards the 400 (or maybe a 800XL as it already has 64K of memory, unlike the 16K 400.)  The 5200 is a bit more of a niche console that needs a bit more expense and effort to get around the controller hurdle, and will never have the large library or expandability of the computer line.  (But I do love it, both as a collector and a gamer!)