The Halo series of bearings are a high quality steel/chrome bearing with metal shields. These bearings have been tested and proven by many racers.
The 3x6x2.5 Metal MR63ZZ bearing is commonly used in the Mini-Z cars and oil instead of grease. Reflex Racing used these bearings to win the Worlds in 2008. Check your manual for the correct size for you vehicle if you are not certain.