I suspect you have to try one out from a friendly dealer. I've had 525ds, 528, E class, Passat and now Golfs.
The best for very long journeys in a straight line was undoubtedly the E class. I suspect it still is.
I moved from the E class to a Passat 170 tdi dsg. That was due to tax reasons. I enjoyed the Passat, and as my journeys were then Heathrow area to Gatwick area on the car park known as the M25 I did not miss the E class.
I then went for a Golf with the same engine and gearbox. As you say, excellent practicality, and easy to park, as well as good handling.I changed that for a GT tdi 150 which I have had since March.
I suspect that had I moved from the E class to the Golf, I might have been less happy.
Find a dealer who will lend you a GTD for a couple of days. I think that is the only way you will be completely happy.