What is the difference (use, effectiveness, etc.) between aciphex (rabeprazole) and prilosec?

Medications: They are comparable medications in the same family of medications referred to as ppis. Major difference is the effect for relief of symptoms in a particular patient. Either drug may or may not be effective for relief of symptoms due to acid reflux("heartburn") or ulcer type symptoms. It is not uncommon for a physician to use different ppis until finding the one which provides the best relief.

Very similar: They are both from the same class of medications so very similar, though people can always reposed differently to medications. Prilosec is available over the counter as well.