I don't think it's so much standing versus sitting so much as how much movement one does throughout the day. A person could be standing in a fixed position which would be far more unhealthy than a fixed sitting position. I remember reading a quote from an extremely old man who said the secret was simple to "keep moving". Movement seems pretty clear to me the most important activity, since it keeps your cells active and fluids flowing. Any state of immobility will eventually put strain on whichever body part is taking the most pressure or work.