Nikon Speedlight SB-900 vs SB-700, which one to go for?

They are both excellent capable flashes that will work seamlessly in TTL or manual mode, both on and off your camera (the D7000 I believe has the Nikon wireless flash remote control system, AWL/CLS)

The SB900 has more features in that it'll automatically colour balance with different gels but unless you really need this feature, then the SB700 will be more than adequate. It also has better selection of beam spread.

The SB700 and SB900 are both expensive thought (£280 - £320) so do bear in mind that if you don't mind adjusting your flash power manually (rather than relying on the, somewhat unreliable, TTL) you can pick up a similar spec (in terms of zoom, power output) manual flash for about £40 (YN560). Also the off camera flash system, while handy to be able to use TTL and adjust flash power from the camera, has it's limitations as its an optical trigger. Although I am a Nikon shooter with Nikon & third party flashes, for off camera work I prefer the reliability of radio triggers - a set like the RF602's can be had for about £18.