The Venice core is the latest of the 3 and has some advantage over the other 2 cores. It supports SSE3 has a revised memory controller and has quite a lot of overclocking potential. However San Diego has everything the Venice core has, and it has twice the L2 cache. The Winchester is an older core. Winchester doesn't support SSE3. Newcastle core's memory controller is weak, older. The Winchester is similar to the Newcastle except that it runs cooler because of lower voltage.
So San Diego or Venice

San Diego should be about $25 - $35 more expensive.
regards.