A lot depends on the individual; physiology, spinal issues, fitness and conditioning. It takes a fair amount of riding for your body to fully adapt to it. The thing is to progress gradually and incrementally so as not to injure yourself. That said, for me, lower is better. My bars are 10cm below the height of my saddle. My wife found lower was better too. We're both ~60 and in pretty good shape. When I see people sitting upright on bikes, it gives me the willies.