The W124 .vs. Jetta...
I rode in the Jetta (my friend was looking for a car in that class). We rode in both the VR6 and the 1.8T
The VR6's ride and smoothness was amazing. If I were to say it's probably softer than a W124 (last 124 I drove was the 500E though)
I don't know about the engine too well. The Jetta's TDI sounds tiny - quiet. The Merc's 2.5TD engine sounds more 'diesel'
But there is no way that I'd not trade a Jetta (if I had one) for a nice W124. The W124 has such a substantial feel to it. It drives like a tank, it's very solid. Maybe it's a fake impression but I really love the 124 chassis. I think it's still the most handsome/distinctive midsize sedan there is. Then it would be the BMW E39.
If you're lucky, some 250D Turbos actually have leather and heated seats
Good luck,