Both systems are widely used in the bearing industry but skate manufacturers have adopted the ABEC ratings. If we're going to talk about tolerances, the fit of your wheels and axles has a much greater effect on performance than ABEC rating. Wheels and axles for inline skates have extremely loose fits that allow you to press the bearings into the wheel by hand. This masks the benefits of a higher precision bearing by allowing it to slip on the axle or in the wheel. Slippage between the mating parts results in energy loss.
Lost energy is lost speed.
Ah yes, the ever so beguiling ABEC rating. A lot of skaters go around bragging about how much faster their new ABEC 7 bearings are than their old ABEC 3's. This may indeed be true but it has nothing to do with the ABEC rating itself. Not unless you are skating at 330 mph, that's based on a 608 bearing limiting speed of 32,000 rpm. Regardless of how fast you plan to go, speed is affected first and foremost by the choice of lubricant.