For $1,000 over the Sport Line, the M Sport Line adds this more aggressive lower front fascia. Unfortunately, far too cold to wash the car. Looks much larger than previous 3ers (because it is).
Proportions remain classic cab-rearward BMW. M Sport Line also includes different rims and rockers. Much less fluid than the BMW, but cleaner body sides than the 2010-2013 E-Class.
The roof line, though not coupe-like (4 Gran Coupe for that), is less formal than the Benz's. Typically technical BMW styling inside as well. Blue trim strip is only on IP, not doors. Why?
Nav screen appears like it can retract, but cannot. Good design for an open view forward, though. The Mercedes has a flashier, more opulent interior. Part of what an extra $10k gets you.
Somewhat idiosyncratic controls mostly easy to use--once you learn how they work. You want the Sport Line or M Sport Line for this driver seat (and a better suspension).
Rear seat roomier than people expect from a 3-Series. Legroom matches E-Class. The 328d's trunk is also large for the class.