If you're comparing a 20-year-old TD to a much younger TDi, you'll likely find that the younger car is going to have less problems. Not always though, our last new Jetta was the worst car we've ever owned. Depends a lot on the maintenance/condition of each car also when you're buying used, and luck.
The two cars are completely different though. The Mercedes TD is quiet, rides smooth for a long haul, lots of room, can carry 1/2ton in its cavernous interior, and is built for the long-haul. Parts aren't too bad for most things. Many used parts are available for the T124. however the 603 engine parts are a little less common used as so many of the diesels are still on the road.
The TDI Jetta/Golf is a commuter; small, rough-riding, and loud inside. It compares well to smaller Korean cars, but not to the 300TD/T124. Lots of people are happy with Jetta/Golf cars, they suit many people's needs and are more economical to drive, it depends on what you need in a vehicle. We drove VW happily for years until the quality started to fall, then moved into luxury vehicles.
The Dodge Ram Cummins is loud, obnoxious, and large. It will haul more than both of the other contenders put together, can haul 3/4ton and pull a large trailer, put a cap on it and its cargo is protected from the elements and somewhat from theft like a wagon. Again it's a complete departure from the other vehicles; larger, louder and much more fuel cost. You could tow the TDi with the 300TD and get better mileage than the Ram.
Most of all I'd start with what you really need in a vehicle. Do you need a small-efficient commuter = VW, or a mid-size classic luxury people/cargo hauler = 300TD, or a small truck to haul with = Dodge Ram.
I've got everything from small VWs to a 454 Suburban around here, each has its own purpose.

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Gone to the dark side

- Jeff