Hmmmm, PNG vs. Fiji....Of course PNG is harder to get to but the payoff is no other divers but you. Some sites in Fiji can get crowded. (We did both on live-aboards)
Fiji does have more soft corals and some really spectacular sites but the reefs, in general, in PNG are healthier and more dense with life.
About equal for sharks but in PNG we had basically 3 dives where they did some basic chumming. No silver tips in Fiji. We only saw fleeting glimpses of Manta and rays at both locations.
PNG had more large schools of jack, batfish, barracuda, etc.....
Macro, again the advantage goes to PNG. For example I don't think Fiji has any pygmy seahorses, not sure.
Fiji was mainly bommie diving where in PNG you had a variety of reefs, bommies, shallows and walls.
So in General PNG just has more of everything and you share it with less people. But the price you pay is it takes at least two days to get there. Given some of the deals Peter Hughes is/was running on PNG I would be tempted to repeat that before Fiji.