Both are excellent choices, and the decision comes down to your preference. The Mercedes-Benz C 220d has a more upmarket feeling and techie cabin, with a more advanced infotainment system that is easier to use. It’s also quite a sharp handling sedan if you enjoy driving yourself.
However, the BMW 3 Series LWB is also excellent to drive, and its longer wheelbase doesn’t affect the dynamics all that much. Moreover, it is far superior in the back seat, thanks to that longer wheelbase, and so if you are chauffeur-driven, it is definitely the better choice.