I do not see any major improvements in the specifications. To be clear, however, I have been considering buying our first SB-900, so I have no actual experience with it. (We have several Nikon flashes, up to and including three SB-800 flashguns.) The existence of the SB-910 might result in somewhat lower prices for pre-owned SB-900 flashguns, which I would like to see.
Canon 5Ds R/7D2/7D/5D/40D/1D2N; Nikon F6/D3s/D700/FM3A/Coolpix A. Lens selection undergoing changes; some favorites: Zeiss 2/135 APO Sonnar, Canon 100mm f/2.8L Macro IS and 135L, Nikkor 14-24/2.8G and 200/2 VR.