If I were in your situation I would wait to see if the 27 model year has the new engines, then wait if they do.
Being on my third Yukon XL, swore up and down I would replace it with another XL. I am starting to lean towards a Sub for several reasons.
Chevy is much better at letting you pick and choose certain options, for instance the trailer/tow package. With the Yukon you have to get either Super Cruise or the Elevation Max package, can't order just a stand alone tow package.
I don't want those stupid rear entertainment screens hanging out in the middle of nowhere, and with the Yukon they come with the Max package.
I don't really care about Super Cruise, for my habits it's not worth the added $$$
I want a sunroof, with the Yukon, again it's not a stand alone option.
I can buy a heavily optioned LT for around $8k less than an Elevation with stuff I don't really need or want.
Not a huge fan of that big screen either. I may be a bit impartial to the Chevy screen though, the daughters 25 Equinox has a very similar layout, but to me the screen just flows better. I realize this is personal.
One top to this, I have a dealer right here in town and one 10 miles away.
The nearest GMC dealer is 30 miles away, if I would need warranty work.
Why does GMC do this?
I love the looks of the Yukon, and I like the looks of the Suburban.
1st world problems, I get it. Just irks me a little that GMC isn't more ala cart with options choices.
I'm sure they have thier reasons $$$