SB-910 used to be like S$640-680, I believe now is below S$600. SB-700 is about S$400?
SB-910 will refresh faster in same situation because you don't have to fire at nearer to full power as the SB-700.
With SB-700 once you got spot meter on the camera it will go TTL and on Matrix it will go TTL-BL. on SB-910 you have full control.
SB-910 lets you zoom to 200m for narrow lighting, more control if you someday do CLS.
SB-910 is not much heavier than SB-700.
If you do large group shot in a dim restaurant, you will want the SB-910. You will need the DOF hence surely at least f/4, then 1/60s to be sharp in case the people move a little, at ISO100-500, you will need SB-910 to bounce on the ceiling.