Definetly the AMD. I think it might be a 475MHz though (not 500). If it is then that means its probably a AMD K6-2AFX processor or similiar. I have one sitting on my floor right now...it's an ok machine. It'll do what u need it to for work/home...just not much of a gaming PC.
Depends on what you mean by "keep and upgrade" ... if you mean "take it to a shop and upgrade" then use the P2 as thats crappier ... if you mean "scavenge for parts and do the upgrade urself" then use the AMD