Whilst the weight of the car is important for towing, so are the aerodynamics. If you have a vehicle with a large frontal area, the less impact towing it will have on stability and economy. The Vito van was the best tow car I've used, with the ML a very close second. I never towed with my 221 S class, but have towed with similar cars and they are were never as good as a van or large SUV.
My current GLE is as nice a place to travel in as my S Class from a decade ago, but has no tow bar as I have a Defender with one. But I know which I'd choose

Just my 2p's worth!