I guess part of this is my own personal opinion and taste.
For me, there is no difficulty in this decision whatsoever. To start with you're looking at TWO generations of difference in technology. I had a '59 finback sedan like the one in the picture for a short time while I lived in Germany in '69. It was okay as transportation, but not a very good driving or handling car, even for it's time. Add to that the fact that I think the finbacks are without a doubt the UGLIEST Benz ever put on the road.
The 123 on the other hand, again is two engineering generations of improvement over the finback. The 123 car is totally refined as compared to the finback. Also it sounds like the 300D is a running, well cared for car. If the 300D has been even barely well cared for, 135K would be low mileage.
That said, without seeing these two cars up close it's hard to really make an accurate assessment, but unless there is something really good about the finback and something really bad about the 123, I wouldn't give it another 30 seconds of thought, I'd go with the 123 in a heartbeat.