Best Flash for D610
The SB700 would be a half step down in terms of power and IR AF functionality. The SB-800 that you have is the best you can do at this time. I have 3 of them and would have bought a fourth but Nikon stopped production to release the SB-900 series that overheat and shutdown completely without warning. The SB-910 may be better than the SB-900 but neither adds significantly more power than that provided by the SB-800 and they do have a wider range of focal lengths that will work for IR AF assist but the difference is negligible for 99% of photographers.