Sitting upright is (more) comfortable only for short rides, but isn't 'physiologically better' for you for extended periods of time; for the simple fact that when sitting upright, almost all of your upper body weight rests on your bum and spine. Also any bumps encountered on the road get transferred directly to your spine which doesn't happen as much if it is at an forward angle - that's the reason 'dutch bikes' usually have springed saddles.