I've owned BMWs since my first new 2002 right out of college in 1973, but I've also owned Mercedes since the first ML in 1997. Mercedes pioneered this segment, but both manufacturers make a good product. I just have stuck with Mercedes for my SUVs (have a GLE coming in 10 days) and BMW for my cars (currently an M4).
My opinions regarding exterior appearance of these two (not that you care
) are that I do like the overall angularity of the X7, but I think the kidney grilles are overwrought, more like the direction Toyota has gone. I like the squared off wheel arches on the GLS which are, oddly, more angular than the X7's. Overall I'd give points to the X7. Interior-wise, the huge GLS's MBUX interface is not artfully integrated, but as in the GLE it's pretty impressive in person to use. No good photos of the rest of the GLS interior yet, but the X7's is very impressive. At least in the GLE Mercedes is too conservative with their interior choices of upholstery, etc.
In powertrains, now that Mercedes has gone back to the straight-6 and added the e-Boost with 48v electrics, it has an advantage on paper in the 6-cylinder models, but BMW's six has been refined over so many years I'm sure on-the-road performance is very close. BMW has nothing to compare with MB's E-ABC suspension, but that's a very expensive $8k+ option that worries me from a complexity standpoint after warranty (but I still ordered it).
No wrong choice here.