As a purveyor of fine flyrod poppers (Jamison Lure & Fly Co.), I can tell you that while I make all sorts of poppers, I still prefer flat-faced cork poppers for a subtle yet effective "pop" for freshwater gamefish. The hard foam, deeply cupped face poppers that I also make are a little overkill for me. In fact, I have seen big bass spook when I pop the foamies.
Sure they look better, and are slightly more durable, but the cork floats better, lands on the water better and I tend to catch more and larger fish.
Just $0.02 from an old popper designer.