I think that seated climbing is definitely more efficient. But after some recent 'gearing issues' limiting my ratios, I've discovered that hammering up the hill in a higher gear works better in many cases.
For me, I've found that excessive cadence limits the participation of the 'pull up' leg. It seems by the time I get that rear leg to bite that the cadence is over. The slower cadence in the bigger gear allow me the time to really pull up and truly use every muscle to climb.
Using the standing method, I seem to get up the hill a LOT faster. But I would still say that sit and spin is more efficient.
I'm 6'3" and I only use 175mm pedals. I would like to try some 180mm in order to get my cadence down while keeping my pedal velocity at the same rate. Then maybe sit and spin will work better for me.