Hey thanks for posting this. Interesting read. Great truck, amazing that it's doing the 0-60 under 5 seconds and according to the article has braking equal to the X5M (from the 2010) test. I am sure there is some acceptable variation in those numbers.
We have the 2012 GL550 and I totally agree, would not compare to the X5/X6. Totally different class of car. With the 3rd row folded, you can fit a refrigerator in there. Or in our case, a behemoth foldable double stroller on top of luggage for road trips and such.
The thing is, when we don't need the strollers anymore, for slightly more cash I think my wife would opt for a G63 instead. GL is necessary for the family hauling with infants/very young kids which come with big strollers and enough changes of clothes to fill a laundromat!
I'm adding the X6M to the GL550 and they serve two very different purposes. For family day trips, the X6M; for longer overnight trips, strollers, and luggage, GL550 all the way.
On the fun to drive factor, the size of the GL550 can't compete with an X5/X6, hands down. I seriously doubt the AMG version will change that.