What is a better RC car a buggy or a truck?

This depends on what you plan to do with it. If you plan to do all off-roading (or close to it) a Truck will probably work better if the terrain is difficult and requires it to be lifted of the ground. A trucks advantages don't transfer over to road tracks to well. They are also good if you have long grass.

An RC buggy is better if your doing a mixture of off road tracks, and road tracks. Buggy's sit rather close to the ground so cant go over large 'bumps'. They can easily do jumps though with a reasonably smooth surface. Long grass will stop them in there tracks though (in fact it doesn't have to be so long because there close to the ground). There advantages do cross over to a road track better than trucks though.

I like Buggy's better for most tasks.