Nikon Speedlight SB-900 vs SB-800 – Joe Federer:
Turn off the Thermal Shutdown feature and you basically have a SB800 with a better interface and some better 'zooming' capabilities.
For me, that is worth it for the 'on-camera' one — not for the remote ones. So my on camera (and backup) flashes are 900's whereas my remote ones are 800's. I notice very little difference in power, if any, between them.