A few months ago I spent hours researching flashes for my D5100. I too did not want to spend any more than I needed to.
End of the day I went with the SB700. And I'm very glad I did.
It serves my purposes for now and if I ever want to go past two flashes (the max the SB700 can control) then my third flash unit will have to be the 910.
Unless you are shooting objects that are a long way from your flash, or are bouncing light off of high ceilings, the 700 should serve you well. If you can afford the 910 and don't mind spending the extra $ then go for that one more power is always nice to have just in case you need it.
The other thing I found is that my SB700 fits into my smaller kit bag, a larger flash unit might not, but that would be bag dependent. I have no idea if the 910 is much bigger than the SB700.
I really looked hard at third party units, some were significantly less expensive but in the end I decided for my first flash full compatibility with my camera was worth the extra $150 or so that I might have saved with an off brand. If you have not learned about Nikon's Creative Lighting System (CLS) look into that before deciding to save $ on a non-Nikon flash.
And listen to the advice of the seasoned folk here, I've just been playing at this hobby for a few months but thought I'd share my experience facing the same question you are facing.