I testdrove a full-spec GL 420cdi. And spent some time inside a R-Class.
The interior of the GL is not exactly the same as the ML.
-The dashboard is covered with leather. Much more upscale, because in the ML the plastic is good but not fabulous. The little wood, well....not very important.
-The doorpanels are leather. In the ML they are absolutely HORRIBLE, a kind of plastic with enormous gap, looks very crappy. Only bad part of this interior for me.
So the two "flaws" od the ML are corrected in the GL. Very good interior.
Concerning the R-Class, I did not drove it, but spent some times in one. I NEVER judge something that I did not touched, felt or tried. You will never see me criticizing something that I did not experienced.
So I spent some time in an R-Class. The dasboard looks okay, not more. The dash material is quite similar to the ML. Climates and Comand too, of course. However, a lot of pieces are made of sub-quality material I found.
The panel gaps are not the same everywhere: the dash, for instance. The heater grille are almost touching the doors, little gap. But the glovebox is 1cm from the door...so very different alignment. Not good-looking. Especially with the kind of wave coming across the dash.
Also the front armrest did not give me a very good impression at the opening, from what I remember.
But it is really the rear armrest. I hated this big thing. Gave me a poor impression. It furthermore was not completely engaged, and was very hard to put in place correctly, made an big noise when engaged...
The levels to fold the rear seats, or to access to the third row also were completely out of place ( in the GL the kind of red textile is a bit surprising, bit feels solid at least...)
All in one, these are small details, but the R-Class did not give me a very good impression. Some details were not really well executed.
I don't know, it was an impression. A lot of pieces I touched did not let me a very premium impression.
The ML or GL let me much more satisfied. I like the very solid double bars in the center console. And nothing gave me a cheap impression, contrary to the R-Class.
Anyway, to each his own! The R was also very comfortable to sit in, the captain chairs are very pleasant, and you had more place than in the GL that effectively does not feel that big inside. Especially the luggage space, with the folding seats, is not very high! Also the knee room is not enormous, contrary to the R. But like I said, some details did make me feel much better in the GL.