Only reason I can see someone getting the regular length is if the XL won't fit in their garage. Otherwise, you're already going full-size SUV, get all the space you can - you'll never regret it.
We had a '14 Honda Pilot. While we enjoyed that rig and have fond memories of it, we certainly don't miss the lack of cargo space. Getting ready to go camping this weekend. Last summer in the Pilot we had the roof-top box completely stuffed, cargo at the feet of everyone (except the driver) and couldn't see out the back window. With the Yukon XL all the same stuff is loaded in the rig - no need for the rooftop box and everyone has clear space at their feet.