Hmm if it were my tank, I would get the 2 1500's and maybe an extra 550 or two. Alot depends on the coral you want to keep. Some may like the extra two 550s' but one might be ok too. Also will depend n how you have your rock setup. You want no dead spots ever, even behind the rocks. Any buildup will cause nitrates to rise which is a bad thing. lol
I keep a low profile rockscape in my 90g so the two 1500's are pointed at the front glass slightly upwards to keep good gas exchange which will also help keep your pH stable.
The way the water flows in my tank keeps any poop or gunk from sitting on the sandbed, it all gets pushed up into the water column to be caught and skimmed.
I would start with the 2 1500's and go from there though. It might be enough flow with out adding the 550's. But if you are gonna keep SPS
for instance you will for sure want more than the 2 powerheads.
Hope that helps.