A lot of people love the 8600.

I am not one of them. I like the tower case design and stuff, but it seems every 8600 I have ever touched was either super finicky or just plain flaky.

The 8500 on the other hand has been nothing but rock stable for me. I have owned about 5 of them in the past all from various sources. Some shady, some legit, but every machine ran like a champ.