Not sure if I did the right thing... from Canon 5D3 to D700?

The answer to your question probably depends on what you want your camera to do for you,

but,

I have a D700 with which I remain as happy as Larry. If I had to lose one of my two FX bodies, it would be the D3s, although it is also a magnificent machine.

Your SB910 will be happy in the D700's hot-shoe, or, of course, you can use it as a secondary, triggered by the D700's own pop-up flash.

Go for it!

--
Richard