So far, the SB 700 was for my needs more than adequate .... have never needed more power. But I suppose the flash but like on the camera, since it can be super wear so in conjunction with a battery grip. When SB 900 was the 5 minutes test quite different.
In addition, the SB 900 provides on a D90 simply oversized out .... on a D3 or D700 is different again.
In short ... 700 can only recommend the SB. For me only one question ... Nikon may in Metz and Co I liked neither the processing nor the menu. Since the SB 700 / SB 900 is already in a different league.
Also it made me extremely surprised at how long the SB 700 uses a rechargeable battery pack. I use Sanyo Eneloops and thus are 400 pictures is no problem. Make although no series photos with flash, which sucks the empty batteries probably faster.