As PPs said, FFI is much closer than RI. But RI includes breakfast, has larger rooms, rooms have kitchens (which we only really use to reheat restaurant leftovers). So ... are those things important to you?
Btw, in the Marriott scheme, FFI is usually the 'value' brand, as opposed to Courtyard and Residence Inns are usually considered 'moderate' while Marriott branded hotels are deluxe and Renaissance/J.W.'s are luxury. Despite the fact that the Anaheim Fairfield Inn is a category 5, it still has the feel of a value hotel so if you want something a little nicer and not budget - go Residence Inn. (If we need more space we go RI and take cabs on the way home ...)