Short Direct Answer
What kind of heartless monster are you?
MDF, medium-density fiberboard, is the formaldehyde-enriched bully, a child of the big, bad Fifties, rolled off in giant sheets from Southern mills, glued together, coming from China, from Indonesia. Plywood is the Middle American of building materials, only one notch up from MDF.
Heartpine. Our hearts already feel better. Heart. Pine. Both good words, fifty-dollar words. Heart–blood, emotion. Heart, the thing that loves the baby. Pine, knotty pine, natural. Heartpine they dredge up from river bottoms, hundred year-old heartpine because it is so expensive, so much in demand.
And you are considering putting plywood down over heartpine? What kind of heartless miscreant are you?
Smack down a nice solid layer half-inch ply if your floors can take it. Think this through, because if you have floor problems, they may not be able to carry the considerable weight of ply plus hardwood.
It’s an emotional issue, mate. Has nothing to do with aesthetics. You want a nice smooth surface. Got kids? You doubly want that floor smooth and splinter-free.