Both have their pros and cons - it's purely a matter of personal preference - and each is stronger than the other in certain design aspects.
For me, personally:
1. This aspect of the BMW's front-end is the most controversial and jarring - there's just too much protrusion from the back of the bumper to the grille:
2. Mercedes' front end is better because it's actually more bluff and less protrusive:
3. Not sure I like the colour-coded AMG bumper inserts treatment - it's a bit too "Abu Dhabi" and less "Hamptons" for my tastes...
4. Personally, I much prefer these aspects of the BMW's design:
The C-pillar treatment has a strong identity and looks very upmarket:
The bumper and door shutlines work well with the sharp, linear styling features. As a whole, I think this area of the design works really well.
I've always been a sucker for a straight beltline and the BMW's overall greenhouse is fantastic - almost concept car-like:
The arm of the door mirror and its intersection with the DLO chrome strip and A-pillar is very neat and expensive looking.
Moreover, the greenhouse and its surrounds is entirely bespoke to the 7er and it doesn't look like that of any other BMW.
5. The Mercedes (again, for me, so put your flame throwers away) is weaker in these areas by contrast:
The rear bumper shutline cuts its way across the rear flank almost intrusively and it's a weird line too - a by-product of car's polarising taillight design, where the line cuts down from the leading edge of light cover and then has to bend toward the rear wheel arch.
It results in a strange interplay between it and the rear door shutline.
The Mercedes' roofline and DLO is very familiar - the design itself is distinctly Mercedes Benz - but then perhaps it's a bit too familiar:
The single arc of the chrome window beading is very elegant but it's also a very similar form to that on the E-Class and C-Class
I don't feel that it's sufficiently set apart from the lesser-model sedans in the line-up. It's an example of one-sausage, different lengths.
6. Overall, the Mercedes is definitely the sleeker and more elegant design when appraised as a whole. The BMW is certainly not as elegant but it is visually arresting and exudes an immensity that can only translate to on-road presence whether it looks good to the viewer or not. The Mercedes has better proportions holistically than the BMW where the ratios of wheel diameter to sheetmetal are in the Mercedes favour. The BMW needs massive rims to visually offset all of its bulk.