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core i5 is based on the nehalem architecture (same as core i3 and core i7), its is a completely different architecture from core2. not to go into a lot of detail, the main differences with core i5 are an integrated memory contoller, QPI (replacing fsb), monolithic quadcores, and turbo boost.

If you asking which one is better, then the answer is core i5. its newer and newer products tend to offer the best bang for the buck in all price ranges. i.e. if your looking at a core2, for the same price the equivalent i5 would be a better buy. and going with the newer platform will allow you to upgrade in the future.