For Core2Duo this largely depends on the FSB strap - in general though - always go for a higher clock as that gives extra bandwidth, tighter timings do help, but not as much as extra bandwidth.
I think tight timings are much more important for the AMD Athlon64 and X2 processors as the memory controller is on the CPU, but I've never had an Athlon64 or X2 system so I couldn't really say for certain.