I've owned both. Generally, the 300D gets less than 30mpg, and the 190D gets more than 30mpg.

The 300D is an E class vehicle- finish, chrome, sound proofing, weight, door thunk. The 190D is not- it was designed during the oil embargo to be a light, fuel efficient, still MB ride, but as an early model line vehicle, it has more plastic, thinner window glass, and not enough sound proofing. Every early 190 I have seen has rust around the jack points.

For winter driving, the 300D has valves that must be adjusted before it gets cold for reliable cold weather starting, and we always used a block heater below freezing. The 601 diesel in the 190D has self adjusting hydraulic valve tappets, and is a much easier cold weather start. Mine has a block heater, but I rarely need to use it.

The 300D with an all iron engine will not blow a headgasket, but the 190D 601 can because it has an aluminum head on an iron block. Typically, it blows between cylinder 1 and the timing chain cavity, so massive amounts of blue smoke spew out. That's happened to one of my two 190Ds.

I rec the 300D for looks and to be seen in. I rec the 190D for fuel efficiency and practicality and fun driving:-)