Paul, the others have pretty much summed it up. I have one of each, and love them both. As others have said the 1500 is more suited for interstate riding and longer trips, whilst the 800 (more like your 900) I prefer for short trips, tooling around. Also, I much prefer the bigger bike for 2-up riding.
I don't think, though, you'll be "out-biked", with the 1500, if you handled the 900 OK