Both stain and varnish can be applied with a paint brush, roller, or sprayer. However, most stains require only one coat and penetrate completely into the wood surface. Varnish, on the other hand, creates a film over the top of the wood. Varnishes typically require at least two coats during application.
Both products reveal the natural beauty of the wood's grain. Varnish creates a high-gloss finish over the wood. Stain, on the other hand, generally provides a satin finish to wood.
Both products will protect your furniture from moisture and harmful ultraviolet (UV) rays from sunlight. However, when it comes to water resistance, varnish will cause water to bead on the wood's surface, while stain will cause the water to sheet off the wood.
A stain fades over time. The reapplication process is to wash the wood surface, allow it to dry, and re-stain. A varnish will fail by peeling and lifting from the wood surface. Often, reapplication of varnish involves sanding or stripping the wood down completely before applying another two coats of varnish.
The longevity of either product has a lot to do with the amount of UV protection. In general, the darker the product, the longer it will last. This is because the pigments in the product provide UV protection. If longevity is your concern, choose a darker stain or varnish.