We have a 2014 GL350, that's the diesel version and I'm still waiting for our Yukon XL. We'll have both. The Yukon is much larger and my wife doesn't want to drive it daily, but we use it for lots of people or moving lots of stuff.
I can say that if I didn't need the space the Yukon offers on a reasonably frequent basis, and if I had to choose one or the other, the MB is by far the more premium vehicle.
The quality of the ride, the controls, the tactile feedback is leagues better than the GM product. The price difference to me seems more like healthy markup for GM than on par engineering and materials. I'm not knocking the Yukon. It's an amazing truck and serves it's purpose but to me, the GL is just much better. Also, my experience with the new Yukon consists of two lengthy test drives and crawling all over it. I sold my 2004 Denali XL a few weeks ago.
My wife came from a 11 GL (the previous generation) and that was nice also, but the new one is much better.
One nice feature it offers is the 360 camera which makes parking such a large vehicle a pleasure. I was shocked the Yukon, even in Denali trim didn't offer this (yet). Seems like a big lapse to me.
We leased this and the prior MB and I got pretty good deals for both. Long term, maintenance is not a huge concern. Prepare for brake pads and rotors at 20-30k miles and big but not crazy service bill every 10k or so miles. We have had no quality issues with either in the time we have had them that I can recall.