I'm in same dilemma with my 08 Duramax. I just replaced the front wheel bearing and brake pads at 160k miles. Last year I did shocks and new tires (last set did last 80,000 miles). The deal is, I paid it off last year and since then I've put over $2,000 into it, with my brother doing the work on my bearings and shocks, would have been $3,000 if a shop did the work. I do believe that you either have a payment or you have maintenance. Now a payment is more and depreciation on a new truck is significantly more than mine now.
That being said, I'm waiting until the 2020 HD comes out. I've thought about getting a 2-3 year old one, but I need to get to a 15 for a better interior and a 17 for more horsepower (mine is tuned with EFI live). And anything 15 or newer, unless there's 100k miles on it, it's not much of a discount from New. In addition, mine is fairly well loaded, I don't have a sunroof or power rear slider, other then that, I wouldn't gain much in a new pickup. With a 2020, they should have a bigger cab and push button start based on what they are doing with the half ton in 2019.
That is just my thought process.
