I'm not a big fan of the XL. I only use it for grilling. If you look at the height, it's the same height as a medium, yet twice as wide. The fire tends to move around the firebox on low and slow cooks because it's 24" wide, yet only 6" deep. Plus, when the plate setter, there's kind of a dead spot in the middle, so I would always have to rotate everything.
BGE has messed with it quite a bit. When they first came out in 2002-03, the lid was even lower. You could put a turkey in it, but only in the middle. Same goes with pork butts--you couldn't close the lid. So they retooled and raised the lid. Since thing they have made the fire box shallower, and they cut notches into the fire ring so the plate setter fits down in it. The old plate setters were so short, you couldn't put drip pans on them. If they would have made it the same height:width aspect as the large, it would have been HUGE and 200# heavier, but it would have cooked like all the others. It's still tough put two turkey's side by side with the plate setter installed unless you have smaller turkey's.
As far as the medium goes, it's nice, but unless you're getting a deal on it, just buy another large. That way all your accessories and experiences will be the same. If I had it to do over, I'd have three Large's lined up.