I've tried both and I prefer Catherine with aftershock. You're only going to get a few kills so the guy that said you are kill stealing is incorrect. The greatly increased damage from this single item is well worth it. What I'm not sure of is what the maximum cool down reduction is, because you are still going to need clockwork, so not sure if it's going over a cool down cap having both items, which wouldn't be optimal, but either way I still prefer it. Don't wait to finish building...