Both oil-based and water-based deck stains have their advantages, with oil-based stains typically penetrating deeper into the wood and providing better protection.
The best finish for an outside deck depends on factors like desired appearance, level of protection, and maintenance preferences, but a high-quality penetrating sealer or stain is often recommended.
It depends on personal preference and the desired look, but solid stain generally offers more protection while semi-transparent stain allows the natural grain of the wood to show through.