These are both crate engines, what do you mean rebuilding one?
I don't know your budget, but looking at Summit, there is a big price gap between the two. just looking at fully dressed assemblies (manifold, water pump, carb etc...) there is about a 4000 price difference in the two, and only 118hp difference (502 vs 620). is that extra 118hp worth 4000 for you, or would you be better off using that 4 grand towards a tranny and rear end that could handle the power? And then save up towards a blower or something you could add later.
oh, who am i kidding, bigger is better, screw these little things, call up Sonny and get something with some balls.
www.sonnysracing
engines.com