In Italy, i had issues to find pellets for carnivorous fishes.. so till the adulthood my polys ate barely only raw food.(avian, fish, shrimp and other sea food meat) They grow healty without problems. My tanks ar filled with plants, mud and algae, also i give them wild alive gambusia when i find them in the rivers, i think that may have helped. Also they surely ate some of the commericial foods that i give to other fishes.
When adult, they becomed very unfussy and stared to eat corydoras made pellets and pellets used in trout industry, that i used for my tilapias.
I really don't know if one food is better than another. Polys are very adaptable fishes, i think they could eat barely everithing. The mix of pellet and raw food seems the more advisable for me... Also because with pellet is easyer the stock of the food and it's easyer to not overfed. They're basically more pratical. But i don't really know if a poly need that kind of food. Maybe feeding with only one kind of meat it's not the best for them but probably the right mix of raw foods may do even better than prepared ones.