The SB-900 is larger so it's better right?
As far as I can tell it seems to have the same guide number, the same power output, a better zoom head in the flash, and a better user interface, but otherwise is identical to the smaller and lighter SB-800 and significantly more expensive. Oh and it's designed for full frame cameras whatever the heck that means. Sounds a bit like marketing rubbish since the camera will identify to the flash anyway what zoom angle and TTL output the flash should have. From what I've gathered the SB600/800 work just fine with the D700 and D3 pair.
Don't think I'd pick it over the 800. Where as the 800 has some great features over the 600.