Front wheel for differential drive RC car?

I'm building a radio controlled car which uses differential drive. There is a motor controlling each rear wheel, this allows it to turn on the spot. The only purpose of the front wheels is to stop it from scraping against the ground. Can anyone recommend which would be the best type of wheel to use at the front, such as ball casters or omni-directional wheels, the wheels need to roll both forwards and sideways. Thanks