hydrocodone is significanly stronger than codeine... 15mg hydro and 120mg codeine sounds quite pleasant, well depending on your tolerance, but yeah, hydro and codeine are slightlyy diferent, hydro being more noddy and codeine being more warm and itchy (you can always take an anti-histamine 30 minutes earlier to prevent any itching, but a that dose you should't experienc too much discomfort...
Codeine is metabolised into morphine by your liver, so you're effectivelly taking 15mg hydro and 12mg morphine... a nice combo