I'm sure this is a very debatable topic, I personally have read negative posts about the SB 900 and its over heating problem.
If you have been through our portfolio you will immediately see that we don't use natural lighting. As a wedding photographer We always use flashes as you cannot always rely on the days lightings or the venues, currently we have 2 x SB910s and 1 x SB900 on PocketWizards which we constantly use at weddings and glamour shoots so we have definitely put these Nikon behemoths through their paces.
The SB900 does over heat a little faster than the 910. However these are Nikons flagship flashes so basically what I'm trying to say is, you shouldn't be firing like mad at a professional level or if you are in the market for the flagship products.
If you are a sports photographer and using flashes on PocketWizards and you don't have an option but to go fire like crazy I would advise the 910 over the 900 but it really isn't as bad as people say and I believe the latest update helps quite a bit.
Other than the over heating differene on the 2 there truly isn't much of a differene, a few button changes as can be seen below. The button changes are for the better and the 910 just seems more refined when using the menu and buttons and has a slight more premium feel, even though the basic shell hasn't changed.
So based on the fact that the 900 is no longer in production, let me give you a bit more insight on the 910 in the field.
This is one serious bad boy. It's big, it's powerful and it can take a serious beating. I had placed my flash on a rough concrete surface at a shoot to give some unique lighting, my assistant not seeing where I placed it walked past and kicked the 910 that it looked like a drift vehicle doing doughnuts. Needles to say it suffered minor scratches. From light stands being knocked over, kicked, rained on, this machine can take it (please don't try this at home).
Majority of flash accessories from the Gary Fong and Lumiquest range all fit the 910. So you don't have to worry about accessories. One 910 is more than ample to power a small sized studio soft box. But remember to make sure your flash is zoomed out at 18mm to utilize the size of the softbox.
Remember this flash has the zoom capability from 18-200mm. For those not so clued up on flashes, this means that at 18mm the bulb is almost flush with the point of exit of light creating a wide spill of light hitting the floor and ceiling. At 200mm the bulb sinks deeper in the casing of the flash projecting the light down a barrel creating a more directional/snoot kind of effect. When you using your flash on camera and zooming with your lens the flash will automatically zoom to compensate for the distance to which you shooting at.
TTL...the part you have all been waiting for.
To be honest I was not a fan of TTL until I updated my PocketWizards software (read the TT5 review)
TTL is basically having your flash on Auto and letting it decide what to do. Remember your flash will not ALWAYS be able to know what you are trying to achieve, so there are times you would need to switch to manual mode on the flash. (Which is very easy to operate)
I shoot on Manual no matter where I am or what I'm shooting and what is great about TTL is that it compensates what your settings are in manual mode, so if you increase your ISO, your flash will automatically decrease in power. And so the same applies to adjusting your aperture.
So in a nutshell TTL is designed so that you have less to worry about and your flash will always try determine the perfect exposure based on your camera settings whether you are operating in Manual, Aperture, Shutter or Program (sorry to those that use Auto, it still works but the D800 doesn't have an option for auto). Since my update on my PocketWizards the TTL on my sb910 is operating as if it would on camera and it is unbelievable.
For those looking for the remote function allowing you to wirelessly trigger your 910 or 900 with your on board flash read my review on the PocketWizards.
I give the SB 910 a 9 out of 10. I have to leave Nikon with room for improvement, recycle times, power and all the technology they have that they haven't told us about that we still waiting for :P