Maintenance wise, they are virtually identical, having the same drivetrain, suspension and cooling system.

The 300E clearly has the rear seat room and wheelbase to make it a "cruiser" for highway trips, its also much more "luxe" than the W201. While seemingly a minor thing, the lower trunk overhang makes getting stuff in the trunk a bit easier.

The 190E has a "sport sedan" feel, very nice balance in twisty roads, excellent turning radius, fits well in smaller parking spaces (width). The M103 six is so smooth in that smaller chassis and you still get that "bank vault" feel when you close the door.