I do not know much about engineering this sort of things, but it seems to me that working against gravity is easier than working against elastic force (as you p

I do not know much about engineering this sort of things, but it seems to me that working against gravity is easier than working against elastic force (as you propose), because the force of gravity is independent of the amount of energy stored, while the elastic force depends on the amount on energy stored (when the air is already pressurized it is much harder to push in some new one). And in general I believe that it is more difficult to build a motor or a generator that are efficient independently of the force they have to work against/with. Just random thoughts, though...