Venice is better than clawhammer, and san diego is better than both of them. Clawhammer is 130nm, and does not support SSE3, Venice is 90nm, and supports SSE3, but only has 512K of L2 cache. San Diego is 90nm, supports SSE3, and has 1MB of L2 cache.
Chris has the right idea, the 3500+ Clawhammer is socket 754 based and has a slower Hypertransport bus(1600MHz) which hinders performance. The 3700+ has a 2000MHz Hypertransport bus which gives it that little edge.