I hate the Phat DS...it looks and feels very cheap, it's BIG, and the screens look shitty compared to those of the DS Lite
once you've played with a DS Lite for some hours, you won't want to change back to the Phat DS...the screens are brilliant and make everything look much clearer...
well, on the other hand, the Phat DS has a way better d-pad, which is larger, more accurate and more comfortable to your thumb..the best d-pad ever on a handheld, IMO
$50 for a used DS is a damn nice price...I'd say go for it, even if you decide to buy a DS Lite later, you will probably be able to sell the Phat DS for good money on ebay