We have a 2012 GL550, it's a beautiful and very luxury truck. My wife loves it. However it's not performance oriented. It's a huge truck with a big engine, and aside from the fact that (prior to the GL63) it was kind of set apart from the other GL's in that it comes fully loaded as standard, no options, no packages ... The engine is really there so you can have enough power for overtaking maneuvers and the like, not true performance.
Its very comfortable but it is boring to drive. If you're looking for a more performance and driving pleasure oriented vehicle, I'd agree that the X5M would be a more appropriate choice.
I've now ordered an X6M for myself and the wife will keep driving the GL550. I'd take an ///M over the ML63 any day of the week.