Both destinations have superb diving, although IMHO PNG, on balance, is superior.
PNG is more difficult and costly to get to, more of a logistical hassle and has far more crime. The capital city, Port Moresby, is a pit.
I would be very surprised if you were not pleased with MG's suggestion of a 4-5 day liveaboard out of Cairns that takes in the Ribbon reefs and Osprey Reef.
Have a dive circus.