Whichever is cheaper... since the 8500 will run 6400 speed with lowered timings, it depends which is cheaper in this case, since they are probably same memory, just different SPD settings...
That is an advantage with Conroe over AM2 designs... Conroe does not require low latency expensive DDR800 for decent performance, since good performance can be had using cheaper DDR533 at 1:1 synchronously