Hmmm... I would think that the volume of unexpanded steam in the power plant boiler would represent a much larger stored energy than the rotational kinetic energy of the generator ( especially that stored in the very slight reduction of rpm allowed).  But these momentary storage requirements are dwarfed by those encountered at night ( no solar) during a period of no wind.,(reply>), Rethinking this, I don't think its the absolute size of the spinning generators in the power plants, but rather the stored energy of rotation per watt ( or megawatt) supplied. I am not so sure the wind turbine generators would lose out to the big boys after all. I would guess most generators in wind turbines are geared up to 1800rpm(60hz), plus the energy stored in the blades may be nothing to sneeze at in the very short term,(reply>), And you would think rightly. But energy stored in the pressurised steam within thermal boilers cannot be used to stabilize the very quick and slight desturbances of power demand and supply. For heavier and longer existent deviations this energy is widely utilised, though.