it might be an oversimplification, but perhaps it's worth going for something which seems to best approximate prey items found in the wild, in which case i'd have thought pinkies were more scarce
By that logic, i'd say keep them on pinkies for longer. Thinking about it, my guess is that the majority of hatchling pythons who survive in the wild are the ones who find nests. Baby mice, rats and birds are underdeveloped and offer the lease resistance to a tiny python.
Even when they're bigger, they've got a better chance of eating a whole nest of babies than they do of eating the mother. Mothers come and go, they fight back, and they run away.
Also, most hatchlings wouldn't survive in the wild, so we're not aiming to give them the odds they get in the wild we're aiming to give them the best possible chance. Just because in the wild they're eating what is basically jelly beans, doesn't mean it's the best thing for them.