i don't think anyone can say one method is more biomechanically efficient, because it depends largely on things like muscle mass, cadence, weight, etc. for a tiny little fella like heras, standing and spinning seems to work well--he loads up his cardio system from a high cadence and gets more oomph per pedal stroke by standing. for a bigger guy like jan, well, i don't know that i've ever seen him really standing to climb.
i think the combo works best, if for no other reason than it uses different muscle groups. i'll resist the temptation to say "the best climbers in the world, lance included, seem to do an awful lot of standing" and instead say whatever works best for you is what's best for you.