Regarding PS4/PS4Pro, going off of the guidelines issued to devs a few months ago. Resolution, FPS and other aesthetics are fair game. The user experience has to remain the same on both systems though. The mechanics/gameplay and the likes have to be identical according to those guidelines. Basically the majority of PC games today with one running at medium and the other turned up to high. Which is understandable when you look at the specs (CPU is no leap) and it is based on the same architecture.
Regarding XBO/Scorpio it's a different scenario, I think the XBO will still be supported for a while by Microsoft and as long as the numbers add up for 3rd party, but in my opinion it would be a waste not to take advantage of the bigger power gap between the two and not let devs loose and shackle them to create to the lowest common denominator. Also I cannot see any room for the current architecture going forward meaning it will not be the same situation Sony are in.
Besides, by the time Scorpio comes around I think the 4 year span (same gap as Xbox to Xbox 360) and the difference in power is justifiable enough to let devs create better all round experiences even if that means pissing people off. Obviously there would be problems to overcome this way, like online cross play and such but IMO it would be for the best.