need a little advice on D750 vs D810 purchase

I have the funds now to purchase a new body and lens. I use a Canon 70D presently. I've rented a D750 with a Sigma 24-105 and loved the images. Am looking at the D810 now. Wondering if I should rent an 810. Here are a few points I'd appreciate comments on:

D750 vs D810 are the IQ differences startlingly different?

Weight, does the weight difference hit you after walking and carrying the 810 for a few hours, as compared to the 750?

What features make the 750 or the 810 the better all around camera choice?

I am planning to own only one body, use it for landscapes, portraits, as an amateur, for fun.