I believe in regards to only the anti-histamine effects, promethazine is more potent. But if you head on over to an informational page on the drug, like wikipedia you'll see Promethazine has some other cool properties on top of anti-histamine, albeit they're not extremely prominent but definitely alter the effects profile. This sets it apart from diphenhydramine a tiny bit.
I wouldn't know which goes better with codeine because I don't take anti histamines recreationally. However I have read that diphenhydramine has been used as, and is good at increasing the effects of opiates, take that as you will. I suggest doing some googling.
If you ever get your hands on Remeron (mirtazapine) and smoke marijuana after taking it, it's a treat. It also works wonders at instantly ending bad trips, and is a more powerful anti-histamine than both prometh and diphen.
