During college I worked at a movie theater that had "E.T." playing. The candy featured in the film, Reese's Pieces, was meant to rhyme...which sort of determines how it should be pronounced.
IMHO, the tiny "fun size" Reese's peanut butter cups are too salty.
Perhaps the reason the egg-shaped peanut butter "cups" seems to taste better is because the ratio between the chocolate and peanut butter is skewed towards more peanut-y flavor.