MoI = Moment of Inertia.
Or: how much do i need to push to get this thing rotating?
In that, the diameter and mass distribution along this diameter are the factors. For every single bit of mass it's calculated per m*r², so the radius (and weight distribution) is more important than the actual mass to see how fast a spinner will spin up, how much energy it can store etc.
That's why a spoked disc of the same weight and at the same speed can store more energy than a solid disc or bar, because more of the weight is further away from the axis.
That's also why i'm always a bit irritated by people giving the weight of the spinner as only factor.

But apart from having some physics knowledge, i am a noob to robot building, so i'll let the actual answers for other, more experienced guys. Just thought this might be an upcoming question anyway, and wanted to make it easier for those who actually can answer to this thread once they do so