Maybe I can offer some advice for you.
I drive a BMW 7 series F01, which we bought to replace my X6.
My wife drives a BMW 5 series GT, which we bought to replace her X5.
Both cars are absolutely fantastic, and the GT is incredibly spacious especially for rear passengers.
However, the difference in ride quality between 7 & 5 is night and day.
The GT is a completely different ride, much harder than you would expect.
Maybe we notice it more because we are in and out of both vehicles often.
Many drivers put this down to runflats, but I also have runflats on my 7 and the ride is fantastic.
I would arrange a lengthy testdrive with your dealer, and see how you get on.
I would guess that you will notice a HUGE difference compared to your MB.
Don't forget that the GT is an incredible car, as long as you find the ride acceptable you will be very happy with one.