Those critters are worse than rabbits! They breed like crazy! Just ask HBB. He'll tell you. I have witnessed it myself. An Sb 600, 700, and a 900, plus an SU 800 call my camera bag home now. No doubt more coming in the future.
Just don't get them wet or feed them after midnight!
With me it was just a progression thing. My SB-600 was my first, then I picked up an SB-700, then got a decent deal on a used SB-900. Funny how it worked out. Now I am thinking of an SB-800 to fill in the gap lol.
You can always dial down the power, so I would go for another. You can't get more power out of a flash than it is rated for. Really just a matter of how much you can spend. The only other reason not to get one is if you need a smaller flash unit to carry around.
I don't know about the rest of the guys but at the moment I just use the diffuse that came with the 910. I would love to get that Gary Fong Lightsphere and power snoot! Now I abbé to google the diffusers you mentioned, I sure hope I hate them! later Bertotti