I keep a some acropora (my favorite and would like to keep more) a brain, three huge torch corals, a pasture of clove polyps, a huge E. quadracolor, other polyp rocks and some softies. Right now I have a Mag 2 powerhead rotating via a Osci-wave, which the polups love, but I think is too much for the acropora. However the size is large and very obtrusive.
You are certainly right that one 6000 plus magnet is cheaper than two 6500's. I was hoping to get by with one 6055, but if the 6000 has a wider stream as you say, that would be more cost effeective than two 6055's. The reason I was looking at the nano's was strictly size. But if two nanos are larger in footprint than one 6000, I'll get a 6000.