The bearings used in Scooter wheels are exactly the same as the ones used in skateboard and rollerblade wheels. Because of this there are dozens of different grades and brands of bearings available on the market today. Most will boast about being the smoothest and quickest bearings on the market, stating ratings such as Abec 7 and Abec 9. But realistically any bearing is suitable for freestyle scootering. For example, you wouldn't be able to notice any significant difference in performance between an Abec 5 and an Abec 7 Bearing until you reached over 100mph! Which, let's face it is highly unlikely in your local skatepark.
Here at Route One, we sell lots of different types of bearings from £8+ all the way up to £122. Generally speaking the higher the price the bearing, the better quality the bearing will be. Above £30 bearings start to have higher grade materials used in their manufacture such as titanium and ceramic. In theory, these higher grade materials mean that a properly maintained bearing will have a longer life and may spin very slightly faster but this will also depend on the environment in which they are being used and how well they have been maintained. Bearings will spin a lot longer with a load placed on them as this what they are designed to do. In reality there are so many factors in what will make a wheel spin faster and smoother for longer, small differences such as bearing rating won't make a huge amount of difference.
The biggest problem you may encounter in purchasing new bearings for your scooter could be the fitting of the bearings. As mentioned before in the Wheels section, if you are inexperienced in fitting bearings in metal core wheels you may like to seek some assistance as they often need a large amount of force. Plastic wheels on the other hand are a great deal more pliable and as a result are very easy to fit bearings into.