Well, weed is more mellow, more happy.
Hash is way more powerful, it completely screws up your mind. I have experienced some serious hallucinations on hash ...
Weed tastes nicer (when you exhale) ... Hash smells nicer when you're burning it (when you're rolling a joint)
My favorite?
I would have to say, a nice big fat ball of hash mixed into a joint's worth of weed ...