Fill that power gap: batteries or flywheels?

Are flywheels better than batteries? I visited a datacentre a while back that had a flywheel to fill the gap between power outages and the diesel generator kicking in. The datacentre manager told me it made sense because batteries - the usual alternative - were expensive to maintain and to cool

"Batteries would need a chiller full time to keep them cool, which means 250kW off the top of our power usage, compromising our density proposition. Also, batteries aren't particularly green – although green is important, most customers won't pay extra for it," I was told.

Now Chloride, a battery maker - oops, a company that used to make batteries but now focuses on UPSes - tells me that this is hogwash and that if the datacentre manager had come to them he'd have found that flywheels have their own issues (of course) and that batteries hardly heat at all as they only do so when working - charging or discharging - and that most of the time they're doing neither.

Which is right? What's your take on batteries versus flywheels? (That's big lumps of rotating metal, not Wolf J. - look it up!)