DpReview takes a look at the key differences between the two cameras (apart from the obvious sensor size) and compares the two to help you decide which is best for you. Personally, I really like both of these cameras. I prefer the body design of the D500, but I prefer the full frame sensor in the D750. What I would really like to see is a full frame sensor in the D500's pro body design, in a similar fashion to the D700 (Which I'm still shooting with!).
Perhaps at Photokina, Nikon might offer us a little surprise?