In my experience, the CPU slot seems to be better as it gives the accelerator faster access to chipmem. My GVP Geforce 030 in my A2000 runs WHDLoad games flawlessly, while the VXL-30 in my A500 slows some games down.
Using the CACHE option in the WHDLoad tooltypes does get most games running at the proper speed, but there are still some games that are always slow, no matter what I try. I know the GVP A530 has the same problems with chipmem access that my VXL-30 has. (My Geforce 030 has 16MB 32-bit fastram and the VXL-30 has 8MB 32-bit fastram)
