Bearings have 4 main types of shields. Metal. rubber, teflon, and ceramic:
Metal-This is the cheapest bearing and the seal does a poor job of keeping dirt out. Best for onroad purposes as you will be cleaning regularly to keep dirt out. Also commonly found in clutch bells for nitros.
Rubber- Restricts bearing movement and spin. However, the rubber shields protect the bearing from dirt. This makes up for the metal bearing’s in ability to protect the inner part of the bearing. This is ideal for offroad though there is speed consequences. Best used for casual racing or RTR kits over the reduced performance.
Teflon- Work great for racers and those who want performance. They block dirt and spin very free. They are like the best of the metal and rubber worlds. The difference between teflon and ceramic is cost, ability to seal, and while teflon offers greater heat resistance over metal and rubber it is rivaled by ceramic’s ability.
Ceramic- Most costly. However, it offers a better longevity and also heat resistance over the teflon. Offers the best spinning ability and also blockage to dirt. These are the most expensive but the best out there. Hardcore racers, and Squirrel, strive for these and then teflon if not available.