I would check out the two links below. The 24-85 is compared to the 24-120 in the first link. The second link is a review of the 24-120. I have owned both and used on my D600. The 24-85 is a good lens, but I much prefer the 24-120 because of the constant aperture, longer reach, and (in my opinion) better sharpness and contrast.
I really can't see much difference overall between the two (I own both). In some cases (a certain focal length & aperture) one slightly edges out the other. So in terms of IQ, it's a photo-finish horse race (pun intended).
That said, I prefer to use the 24-120 overall because of the longer tele end. I keep the 24-85 as a back-up.
I hate when people ask me what I see myself doing in 5 years...... I don't have 2020 vision!
I'm surprised nobody has mentioned size and weight. These are important to me - I'm sensitive to weight slung around my neck. The 24-120 is over 200 grams heavier and about 20mm longer. Of course if you shoot at 120mm much then these differences may not matter.
The 24-85 is only faster (f3.5) at the wide end. The 24-120 f4 is faster (brighter) through most of the range. Although only 1/2 to 1 stop faster through most of the range it does allow for reduced depth of field which improves subject isolation over a f5.6 lens I find the extra range at the long end to be quite useful. Probably using it in about 20% of my shots. Better than carrying another lens.