You have to store both. Also, you can't vaporize much air at low temperatures, which we aim for. You have to hold onto water for its sensible (no evaporation) heat capacity.
1^m3 of 300 bar air = approx. 30 kwhr.
Approximately 1/4th of that in water storage will hold the heat.
We're aiming at a daily loss of 1%, but it caps out at 10% relative (~7% absolute) because the Energy Out/Energy In is proportional to T_exp/T_comp (in an absolute scale) -- if the temperature drops to ambient, T_exp is only slightly lower in an absolute scale.
