I first bought the Nikon SB-400 for use with my D7000. It is nice and small and has the advantage of being able to "bounce" the light if the ceilings are low enough, however it will not swivel, so using in "portrait" is not very practical. The main problem with the SB-400 is it's not much more powerful than the built-in flash. I sold it after a short period of experimenting.
I now own the SB-700 and am very happy with it's performance. It tilts for "bounce" flash and even "swivels" so it can be used in "portrait" and other odd-ball situations. It is more powerful than the SB-400 and has many advanced features. It is a good compromise of size versus power and has all the features needed for "general" photography.
If you need the most power available, do a lot of studio shooting and other advanced photography the SB-900/910 would be a better choice. It is larger and heavier than the other two and is a LOT more expensive but has the most features. Probably its biggest advantage over the SB-700 is it can control three wireless groups where the SB-700 can control only two.