I have to jump in since i was someone who wanted the Expy and ended up with the Tahoe premier. First of all, looks, the tahoe is the best looking full size SUV ever made. The 18 expy is not. To me it looks like they took the FLEX and just made it bigger. Its a large station wagon. Reminds me of my 2010 Sequioa which looked "bulbous" and not attractive. That's the looks part of it, but my all black tahoe still turns heads.
on the Ecoboost. Sorry, but like others have said, it was fixing a problem that never existed. MPG is lower with the expy. I get over 24 mpg on my tahoe under 65 mph. That's amazing to me.. it gets better mileage than my half size commuter Nissan crossover which struggles to get 20. The power of the 5.3 is very good too. I tow a big wake boat and the tahoe handles it just fine and WITHOUT squatting in the rear.
The interior is quite classy on both, thats a wash to me. Now, i was a ford guy owning plenty of F150's.. i was waiting for the new expy only to be disappointed. I AM NOT A GM guy in fact i swore them off long ago due to reliability. I thought i would give them one more chance and did.. I have 15k miles on my tahoe and so far its been great. Some of my F150's were disasters when it came to reliability especially the 5.4L.
i even rented a new Expy on a business trip and to be honest, there isn't anything i wish it had that my tahoe doesn't or vice versus. I did notice MPG however, and the ford was harder on gas. I also didn't like the shifter knob. I could get used to it, but just don't care for that. The ford will have more room but will be less sporty, that is for sure.
the last thing, when i got my SUV the wife wanted a new car. We got her a lexus and when i was at the dealer they had a reliability sheet sitting on the desk that showed Lexus was 2nd overall. Chevy was 5th, and GMC was way at the bottom. I will say, skip the GMC and get the tahoe. I know, made by same company, same everything but i also hear more problems with denali than i do with the tahoe and the tahoe costs less.