What are you going to be using it for? Also are you just upgrading the cpu or the whole system?
if your just doing moderate gaming, web browsing, or word document Core i3 would be better.
If your doing heavy gaming, redering, video editing, Cad, ect. Core 2 quad would be better.
Although, the Core 2's (duo and quads) are EOL and Core I's (I3, I5, and I7) have taken there place.
So unless you just doing a cpu upgrade on a current motherboard, it will be much wiser to save up the money for a new Intel Core I"x" system or an AMD Phenom II or Athlon II system.