The Halo series of bearings are a high quality steel/chrome bearing with nol shields. These bearings have been tested and proven by many racers.
The 3x6x2 Metal MR63 bearing is commonly used in the Mini-Z cars and has no metal seals (unlike photographed above). Reflex Racing used these bearings to win the Worlds in 2008. Check your manual for the correct size for your vehicle if you are not certain.