My geekbench scores improved by about 15% when I upgraded. SL has been a fantastic upgrade IMO.
xBench scores go down by a good 20% on Snow Leopard.
I would say wait until Snow Leopard is ready... 10.6.5 or so. I am not happy with Snow Leopard on my rev C. Some apps don't work or aren't supported. In addition it runs hotter and slower.
When Snow Leopard is fixed, and 64-bit kernel is really working, AND apps actually use Grand Central Dispatch and OpenCL, AND OpenGL is fixed, then the updates correct all of the problems, THEN and ONLY THEN should we go to Snow Leopard.
I am pretty disappointed in Apple for its release of 10.6 that isn't ready. In the future we will have a reason to switch to SL, but for now Leopard is more refined and works perfectly.