Thanks for your responses. However, I think my question remains unanswered: What's the difference between RA and PNG, and which do you recommend?
Actually the issue is not RA and PNG really. I Have spent considerable time in both areas.
RA in particular around Cape Kri was known for diversification of marine life however this has changed recently with Se Misool taking over the reigns with being ONE OF THE MOST richest areas (NOT THE MOST).
Prior to the work done by world renown ichthyologist's like Gerry Allen the the area with the most diversification on the planet was Kimbe Bay. RA has now joined the club.
In RA and here is there are difference between people but IMHO the southern area (MISOOL) of RA is certainly the best for divers with numerous fantastic dive sites.
It is not that one area is better than the other, the question you need to be asking is what do I want to see. If you want to see some sharks, schools of jacks, barracuda, great reefs, and some critters then you do a LOB trip that does Fathers and Witus Islands.
Most people will not notice the diversification of marine life (as they are not ichthyologist's) and as such do not appreciate the RA for what it is.
Both areas can be difficult to get to (flights)
Both areas are seasonal
If you want macro life the either Milne Bay or Lembeh, or even Secret Bay in Bali
Corals and reefs in Kimbe bay and fantastic (Susans, Kirsty Jaynes, christines, North Emma and Ottos being some of my favourites
Dive sites in Southern RA are fantastic as well (Boo, Kaleidoscope, The Net, Fiabacet, Warakaret, Wagmag being some of my favourites, )