I was in the same position a few months ago comparing the same two models. I ended up with a 2018 limited Max 4wd with the 302a package and 22 inch wheels, rear entertainment and captain chairs. It was hard to find but we drove a few States away to get it. I have three kids and two big dogs so captain chairs were on our list since the kids can walk right to the back row without folding a seat and dogs can lay on the floor with no problems. Once you get the 22 inch wheels, the CCD (continuous controlled dampening) is standard. The shocks basically self adjust to road conditions for comfort and control. They also stiffen in Sport mode and are noticeably different. My main reasons to go to the 2018 4th Gen over the 3rd Gen Navigator was 1) 10 speed auto really helps with drive ability and performance and economy, 2) the Gen 2 eco boost engine that’s in the 4th Gen has double injection (port and direct) and eliminated the engine intake valve carbon problem on earlier models 3) ride, control and comfort of the 4th Gen is very improved, 4) body of 4th Gen is aluminum so rusted rockers and doors are not a problem and lastly, insurance is in some cases much lower on the newer models with crash detection systems so something to ask your insurance company by giving them a vin number of a 3rd and 4th gen to see if there is a difference. I did all the services my self. Transmission, transfer case and brake fluid, spark plugs and filters should be done around 60k. Ford says 100k but that’s pretty stretched out. Next on my list is the front and rear diff fluids. Make sure to use Ford specified fluids since they are very specific to the trans, transfer case and diffs and they do not cost much from the dealer. Hope that helps!