Update after I have 5-6 results from both machines. The two VMs are identical (ubuntu 64).

OSX 10.6.6 w/ dual X5365 (3.0 ghz): 52 c/h

XP64 w/ dual X5355 (2.66 ghz): 40 c/h

Not sure if the difference is purely due to CPU speed. That doesn't seem likely to account for such a large difference. Perhaps the underlying OS of OSX is more efficient in some way.