Skip the Yukon XL or Suburban. Their third row seats are flat on the floor meaning your knees are raised way up in the air. It is VERY uncomfortable and makes you feel like you have no leg room.
If you want the roomiest third row, I would go with the Acadia, Traverse or Pilot. All three of those can fit three in the third row and the second row slides forward for more leg room. The Acadia and Traverse are however a lot longer than the Pilot which means there is more trunk space when the third row is up. The Acadia and Traverse also come with captains chairs which reduces seating to 7, while the Pilot is only available with a bench in the second row. The Acadia/Traverse will end up being a lot more expensive than the Pilot which tops out at only $42,000 compared to their $50,000+. I know two people with Pilots, a 2012 and a 2013 and they both love them; I can't speak for the GM twins.
Pilot
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Acadia/Traverse
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