The question is
D700 versus D610. Both are full frame cameras and both use CMOS sensors.
The D700 is a semi-pro quality, 12MP camera from 2008. Very good for its time.
The D610 was introduced in 2013 and is a more entry-level, consumer grade 24MP camera. Also quite good for its intended customer.
A lot changed in five years time. Features and capabilities "trickled down" to more entry-level models. New sensors were developed and incorporated. Besides double the resolution, D610 also has greater color bit depth, wider dynamic range and higher usable ISO (tho essentially the same range).
D610 has a few less AF points, but some of them are "f/8 capable", meaning that more lens/teleconverter combos are possible. The D700 is "f/5.6 limited... I.e., none of it's AF points are f/8 capable.
They have similar continuous shooting rates... D610 slightly faster with 6 frames per second, D700 with 5 frames per sec.
D700 is more robustly built with better sealing for weather resistance. It also has a higher specification shutter..... 1/8000 top speed compared to the D610's 1/4000.
D610 is lighter weight and a bit smaller.
Several responses mention D750, which is considered a more direct update from D700.
Both D610 (2013) and D750 (2014) have been in the Nikon lineup for a while now and are due for replacement soon. Some Nikon "roadmaps" suggest they will be phasing out DSLR models, in favor of more mirrorless models in the Z-series. It's hard to say for certain whether there will be D610 and D750 replacements or if a single model might replace both. But, since these models may be replaced in the not-too-distant future there may be some favorable sale prices on them. And because they have been on the market for five + years and are popular models, both D610 and D750 are pretty widely available used or refurbished, for some possible savings. Prices for used and refurbished will further drop, if and when a replacement model is announced.
A few more detailed D700 vs D610 and D700 vs D750 comparisons:
https://cameradecision.com/compare/nikon-d700-vs-nikon-d610https://cameradecision.com/compare/nikon-d700-vs-nikon-d750https://www.apotelyt.com/compare-camera/nikon-d700-vs-nikon-d610https://www.apotelyt.com/compare-camera/nikon-d700-vs-nikon-d750 https://www.dxomark.com/Cameras/Compare/Side-by-side/Nikon-D610-versus-Nikon-D700___915_441 https://www.dxomark.com/Cameras/Compare/Side-by-side/Nikon-D750-versus-Nikon-D700___975_441 http://snapsort.com/compare/Nikon-D610-vs-Nikon_D700/detailed http://snapsort.com/compare/Nikon-D750-vs-Nikon_D700/detailed