I personally am just using vinyl overlay by itself. I haven't even completed the cab, but it's already survived a couple bumps and bruises without any problem, and I've seen alot of positive reports on it here. Generally the feedback I've gotten is that if you get the heavier vinyl overlay, you don't need a cover, but if you get the thin stuff or print your CP on paper, then you want something on top like plexi. Most people don't bother with doing both heavy vinyl and plexi, from what I've seen.
If you are just going paint, I would definately look into a plexi cover, however.