Funny that you equate sensor density with professional needs. Nikon's most expensive flagship camera, the D6 is $6500 and at 20 mp has less than half the resolution of the D850. There are many other factors that make a camera desirable for "professional" use - and megapixels, touchscreens, dual card slots, etc are not that important to a pro.

The D850 has the widest dynamic range, with 46mp the most flexibility when cropping provided you have exceptionally good lenses, together with acceptably fast shooting, ideal for birds and wildlife, and a lot of other features that may make it a candidate for consideration. The D850 is currently on special for $2500, the D750 is $1500. The D850 having been released in 2017, is newer than the D750, which was released in 2014, and has since been superseded by the D780.

Bottom line if you like to shoot landscape and wildlife and can justify the higher cost of Nikon's better lenses, the D850 may be the better choice. If you are a very casual shooter, not averse to compromising image quality with a mediocre lens that has a 10.7X zoom (the 28-300) the D750 would be a better choice, since the extra resolution of the D850 will not only give you no real increase in sharpness and detail capture, it will reveal the flaws inherent to the 28-300 and other less costly consumer/prosumer lenses from Nikon and others.