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?
Yeah, I wonder what kind of back pain is that. On a dutchie your weight goes right onto your lower back (just like when sitting down), and it's the lower back what you use for your pedal strokes. Not great for extended periods of time.
On a trekking bike your weight is more distributed, since your torso is sort of laying down.
In my experience: It's more likely to be some detail about your two positions than the coarse category.
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.
You are correct the upright position on a city bike is harder on the spine! That is why upright bicycles usually have a real mattress sprung saddle not just a piece of cheap plastic or carbon!
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?