I did some benches and 10.6 was slightly faster than 10.5. I think I posted it on the 10.6 thread when it first was released.
But, like OverClkr says, some people have issues with it.
IMO if 10.6 is having issues, 10.4 is the next best. 10.5 doesn't really seem to do much (I think it was mainly to fix some loading issues for CoD or BF:BC2 or something)