my (limited) experience is the opposite. I have more back pain after a ride with my upright, dutch-style ride, than on my sporty trekking bike, that has a more inclined position and I ride comfortably for longer and on more irregular terrain. Maybe the vertical position is worse to absorb road vibrations?
Could be. My point of reference is The Netherlands, where we have good cycling infrastructure and the roads are typically very smooth.
On a trekking bike your weight is more distributed, since your torso is sort of laying down.
In my case, a change of saddle made a great difference. The two saddles weren't particularly different in price or overall look, but one happened to fit my body much better than the other. Luck.
Though, if I could go back a few years in time, I would just buy something like an Urban Arrow. We had one last holiday and they are awesome for transporting kids + groceries.
It doesn't make much sense, in a way: If I have a trailer and 150-200kg of total weight (and I have as much every couple of weeks), why should 5kg more or less matter? But it matters: The bike feels better when it's not that heavily loaded, ie. most of the time.
What matters most of the time, matters.
That said, Dutch-style bikes are not for me. I like trekking bikes, which often have a good trade-off between speed, weight, stability, and lower maintenance than most road bikes (especially with belt drive and a gear hub). I also have a hybrid bike, which is lighter and faster, but I don't really like the amount of maintenance it requires (regularly degreasing the derailleur) and it feels less stable than a trekking bike.