I was in the same boat as you a few months back, do i order an Expy or get a 2017 Tahoe. I went with the tahoe. I hope i don't regret it. But here is my take;
The GM was far more expensive in 2017 than the 2017 Ford. The 2018 Ford expy will be far more expensive than the GM from what i am seeing. The platnium trim is up near 80k. Sorry, but won't take that kind of risk for a first year model.
Again, this is a first year model. It should be fine, however, first year anything you have to expect some issues to pop up.
I was unsure about the Ecoboost motor. There seems to be issues with gas getting into the oil causing sludge. Now, they may have fixed it with the new EB, but there is information saying that the motor will have issues on some vehicles around the 100k mark. That will either kill resale or cost you money to fix. I am just not sold on the 6 cyl with turbo yet.
The ford also is not that attractive. To me it looks like the ford flex. The expy hasn't been attractive since the early 2000's where it actually looked nice. The GM vehicles turn heads. I have had so many comments on my black tahoe i lost count. It just looks awesome.
gas mileage, the ford, even thou claimed it matches the GM in the real world doesn't seem to be true. I can get up to 23 mpg in the summer with my tahoe. My neighbor has the 2017 Explorer with the 3.5 and he hasn't seen over 20 yet on highway. Said he is averaging about 18.5 when he babies it. When you are in the turbo it burns gas.
Ford will have a better tow rating than the 5.3L GM, but the heaviest i tow is about 6k lbs. If i ever go above that i will get a pickup with a diesel which is meant to tow more. My tahoe is rated for 8300lbs and the ford can get up to 8500 lbs i believe so not much of a difference.
All that being said, i have owned alot of fords in my life. 2 Expys', 3 F150's. This is my first GM with the Tahoe. Some of the fords were fantastic, some not so much. My 2006 Liarat 150 was the biggest POS i have ever owned with the 5.4L motor. It left me stranded 2x in under 65k miles. I swore i wouldn't buy another. Once i stranded in the winter outside of cell coverage. Fuel injectors, electrical issues and terrible dealer support. I am not saying the Expy will be junk, quite the contrary, just saying take a look at the big picture.. you will be spending alot of money.