Really no noticible difference in performance between the two chips.
A 64Mx8 chip holds more data, and theoretically data travels faster once inside an IC because the connections have a higher degree of integrity. With this said using higher density chips should be faster. Unless used in any volume together, the difference is negligible.
Also, how the chips are used can affect the performance of the module in a particular system. Comparing a 512MB module built using 64Mx8 with the same module built using 32Mx8 chips, the difference is the number of chips used; 8x 32Mx8 chips or 16x 64Mx8 chips. A 16-chip module usually has two memory banks, whereas a module with only 8 chips has one memory bank. This can affect memory performance.
Normally, double-sided modules have better performance on motherboards, but it's not always the case.
hope this helps.