Loving my new D500.
I struggled with this same choice a few months ago.
I also tossed into the decision a D800 refurb and a few other options crossed my mind.
I see the D500 as the "Flagship DX" - creme of the crop camera. In the end it was my choice - a few dollars more than the D750 but not a lot and at the price point what is a few extra $100 dollar bills.
The odd memory card is my least favourite part.
I don't miss the built in flash - shot an indoor party with a pair of F 1.8 lenses and the D500 worked just fine without a flash. (not that the D750 could not do the same).
The speed of focus and burst rate is great - shoot a series of shots of a child at play or dog on the run, pick the best shots (or make a funky gif if you're so inclined).
I have a D610 which is meets my FX needs well enough at the moment. Might still get a D8XX someday - or maybe the D750. But for my compliment of cameras is working for me.