Keep in mind though that thicker might not always be better. Polyurethane is more prone to cracking after 4 or 5 coats. In this case, varnish might be the best choice. The durability of a wood finish is an important part of protecting the wood beneath. A durable finish is more important for a tabletop than a mantle or picture frame.
Durability has more to do with the chemistry of the finish than the number of coats.
For example a single coat of polyurethane is more durable than multiple coats of a water based finish. It ll probably surprise you to learn that the ease of applications is one of the key factors to achieving a professional finish. Slow drying finishes like polyurethane and varnish are relatively easy to apply with a brush. However, their slow dry time leaves them vulnerable to dust landing on the surface and leaving spots. Faster drying finishes can be difficult or impossible to apply without a spray gun.