Pullovers, while still a pec move, are more of a Pectoralis Minor/Lat move than a Pec Major Move(which the dumbbell fly is). I can’t say that either move is very effective but I do Pullover Variations because I like them more. But whenever I think about how effective an exercise is in my routine, I usually ditch the Pullover in favor of a Pec-Deck Fly or a Machine Chest Press. Hammer Strength Machine Presses are a good Press/Fly hybrid. At least I think they are because I don’t feel anything but chest on the Hammer Strength Wide Press.
So Pec-Deck or Chest Press Machine (Preferably Hammer-Strength or Magnum Brand)