Flew to Hawaii in October. Took the carseat onboard and checked the pack-n-play. Only 1 flight attendant checked for the FAA approval. Ours was fine. Our son is used to the pack-n-play, so we thought it would work. It did, but at the first hotel we stayed at they had a crib that we used. It worked about the same as the pack-n-play.
I think you would probably be fine doing either. We just didn't want the extra expense of renting the carseat and crib in Hawaii (all total would have been about $200 for a week). Also, since we were able to pack light--it was Hawaii, shorts & bathing suits, we didn't mind carrying the extra items. That would surely be a consideration, especially since airlines are beginning to charge for baggage.
Oh, one last thing as a side note to think about--our son is 3 y.o. 30 lbs. Our car seat is one of the ones that can be used 20-100 lbs, so it is a bit on the bigger side and it sort of reclines. It put my son's feet right up to the seat in front of him. As soon as the man in front reclined, he started kicking at that seat. The more we tried to get him to stop, the more it became a game. The flight to HA was miserable because of this. We took the red-eye back and he slept the entire way, no kicking.