There are many interesting things in CEP but 99% of them won't be available in Original Campaign. If you're not a module builder than you only need it to play modules that already use it. Adding 4GB of haks just to use couple of new heads/robes doesn't make much sense for me.
Project Q on the other hand contains many improved creatures (new goblins, undead, orcs, ogres, spiders, etc...) which make the game looks much better. I saw OC files with PQ installed somewhere on the Vault, but I can't find them right now :/
Anyway I think PQ is better choice for OC than CEP