A lot of my stuff is on a tripod with a long lens - I have both the D700 and D7100 as well as a D300
For birds I always seem to grab my D7100 ….. no matter what the conditions ….. based on the thousand of images that I have taken
For general shots in any light I use the D700 with say the 105mm f2.8VR, 70 200mm VR and say a 50mm …… the D700 seems to have it's own style …. which I find attractive
I find the D700 cleaner in general shooting no matter what the ISO …….. but with crops I prefer the images from the D7100 …… but with crop you get noise and the tighter you crop the more noise you get especially in the blacks, bg and shadows …….. the D7100 can respond well in post to noise reduction ……. but then again you loose detail
I'd say it depends on the type of photography that you are interested in - the D700 is a marvellous camera and (I find) it quite different from the D7100 ….. they are both good in their own way …….. but if you want super high ISO performance the latest technology in say the D750 or D810 has improved this significantly, (so I have read and so I am told).
If my main interest was NOT birds and insects ……. I would favour the D700 ………. just my opinion based purely on the images I have taken and from using both on an almost daily basis
If the OP is looking at prices, the D700 has stood up well in the used market, but the D600/610 new can now be had for a really good price …… check out the high ISO performance on that ……. it has a far more "modern" sensor