I have both and I am keeping them. They are both good in their own right and price you pay. The SB910 is better then the SB900 in that the over hear issue is addressed. That is not to say the SB900 is not good. The over heating is only an issue if you fire a lot and in full power for long period of time. For casual shooting it is fine. I had the SB700 for a long while when I got off the SB900. Did not need anything larger or more powerful. I got the SB910 simply because it came FREE with the D4 when I bought it. Better be free.. paid a bomb for the D4 heheh If you want the most powerful flash from Nikon, that would be the one to get but if you don't use flash as often, get the SB700.
It's a pity I lost the URL link to a website I once surf for a long time to look at the creative use of Speedlight flash. What was amazing at this site which put people into must-buy-best-gear-but-out-put-bad shots to total shame. This are a group of photographers who only shoot using SB-400 alone. The photos are totally amazing and most are just keen amateurs non pro shooters.
Wish I can find that site again to post it here.
With any gear we buy, buy gears you will really appreciate and use. Not buy to feel less 'inferior ' standing next to a photographer who happen to have better gears then you. No end to that. Compare your ability not gear. Gear is easy to buy by anyone with money. Talent and creativity is not.