You don't need DDR2-8500 even if you do overclock. Running memory at the highest speeds doesn't always yield the best performance. It is better to run tight timings at the highest possible speed.
Both are great companies, i would just get whatever is cheaper. I own the Mushkin XP2-8500 and it is great. However, i usually recommend the Crucial because of the price right now. Both are essentially the same ram. Both use D9's. So just get one or the other of DDR2-800