Whether sitting or standing is best depends in large degree on BMI. Lighter riders will do well standing, while heavier riders will wish that their butts were supporting more of their weight. As per comments above, of course training makes a difference also: one gets better at what one trains to do. With a high BMI of 24, I do better sitting on a long climb and standing on a short pop-over. On long climbs, I'm best at a cadence of ~80, seated with my hands on the tops, back straight, and elbows well bent. I try to gear so that my cadence won't fall below 70 if I can possibly help it. My legs go bye-bye below that.

Regardless of whatever lab testing of the not-me may prove, climbing at the same speed, my heart rate standing is higher than my heart rate seated. Heart rate is a pretty good indicator of which energy sources are being accessed.

Be all that as it may, seated and standing use slightly different muscles, so changing off between these two modes may be helpful on long climbs or long rides. Hence it's good to train both. In any case, it's my practice to stand for one minute out of 10 on really long hard climbs. Even though that may redline me temporarily, for me it's still worth doing.