Peter, if you're reading articles on the Internet, some of them may have been written before they started restricting the number of divers on Sipadan. Currently there are five resorts on the island: three clumped together at the Drop-off and two others elsewhere. In total they accommodate around 80 divers. "Barely an inch of beach that hasn't been developed" simply isn't true.
I would also disagree with the comment that the dive ops "care little for the environment". There is certainly some rubbish on the beach, and probably there has been some damage to the coral, but it isn't the dump that you're describing.
We saw far more turtles than I can be bothered to count. Guests aren't allowed to walk around the beach at night, so that turtles can lay their eggs in reasonable peace (although it is possible to do a guided walk with the Turtle Man and see a turtle laying her eggs). All the eggs are collected and reburied in a hatchery; the young turtles are then released into the sea at sunset (so they will head towards the sun and leave the island). I know there are some issues with turtle hatcheries; however, we were told that if they don't collect the eggs, the young turtles tend to hatch at night and head for the lights of the resorts.
Sipadan has impressive soft corals, but IMO isn't a spectacular hard coral destination -- doesn't really have the right topography. For macro, you'll want to take a day trip to Mabul or Kapalai. Takes about 30 minutes by boat and both islands offer muck diving with the opportunity to see many strange sea creatures. Sipadan's main attractions are the turtle and shark encounters, schools of barracuda and jacks, pelagic fish, reef fish and the vertical topography.
Are you a photographer? There were many photographers and videographers on the island when I was there.
If you don't want to go, send me your money and I'll go on your behalf! Seriously, though, PNG also sounds pretty impressive... hard choice to make.
Hope that helps,
Zept