Hi,
I would suggest the 6" drop. A little uphill is OK, but downhill is much worse, as in a panic stop, the nose of the RV will dive, the rear might go up say 4" and if to far, the car can go under the RV! as the hitch bar goes sky high, and the nose of the car is pushed downward.
Also fill the fresh water tank, and see how much the back drops. My fresh water tank is in the basement, under the rear bed, so it will effect the hitch ball mount height. My grey and black water tanks are just forward of that tank, and everything seems to stay about the same as I camp and fill the other tanks with the fresh water. But when everything is empty, it takes 800 pounds off the back of my RV, so I would imagine it goes up 1/2" or so?
Good luck,
Fred.