I sent out an email to Jalopnik (automotive website for those who dont know) asking about the 10 speed and what information they had. This is the response I received:
"Going from a six-speed to a ten-speed is not likely to yield a massive fuel economy benefit. Look no further than the F-150 for proof of that (Ford and GM co-developed the 10-speed). The '17 model F-150 3.5L EcoBoost does manage about 1 MPG better than the '16, but that's also a product of engine tweaks, not just the added gears.
I drove a 10-speed Raptor recently, and I thought the transmission was decent, but a bit quirky to shift at times. "
Not a ton of new information but confirmation that the 10 speed isnt likely to increase MPG by a whole lot, maybe 1 mpg.
So now it comes down to preference and price. See what sort of offers GMC announces to try and move remaining 2017 BUT have a vehicle that is technically 1 year old already OR wait and get the 2018 but roll the dice with the reliability of the 10 speed transmission.