A basic thing to understand about electricity storage is that there are very different needs for different kinds of storage. Flywheels from what I understand are for very shortterm storage needs, i.e. balancing out shortterm fluctuations in electricity use vs. generation. But they're unsuitable for any kind of longterm storage, because they loose power over time.
In the long run with a high-solar-high-wind-scenario we'll need some seasonal storage to get us over a couple of weeks in some circumstances. This will need some storage that doesn't loose power over time.
(FWIW I have no idea if these gravitational storage techs will play any role in that, and one can be doubtful about it.)
Basically flywheel cannot store energy for a long time. It needs to be constantly used to be effective.
Megawatts measure the peak discharge rate, not that amount of energy stored.
For a fixed number of dollars of investment, gravity will store far more joules of energy.
A weight you can just hang there
I know Pennsylvania and New York both have 20 MW storage systems that takes up a few acres and are relatively cheap per unit of storage.