Without a doubt PNG has it all. Having dove both we found in Palau you expect and see sharks, rays on just about every dive. We went to several spots in PNG that were touted Manta Cleaning Stations and got skunked, that happens. In Palau every site that was advertised to have Mantas did, often 3-4 and we ran out of air before mantas.
All that said if we were to repeat either of the sites, PNG would be it.
All that said if we were to repeat either of the sites, PNG would be it.