Your weight is no advantage in that case as you have to lift it up every pedal stroke. It's good to use some different muscles occasionally but, for most people, standing up on a long hill is tougher than seated spinning provided you have sufficient gearing. Granted most riders don't have adequate gearing to spin up a 12% hill.

For the OP, if you're taking 20min to go up 400ft I think you'll find it much tougher on the 12% hill. Probably better to wait until you can get your time up the 400ft hill under 10min (about 770VAM) before tackling the 12% hill.,