I was disappointed that you did not include Deck Defense (permachink.com) in your deck-stain testing. Any coating is better than none, but Deck Defense is water-based, while most of those sold at the big box stores are petroleum-based and fail quickly, resulting in another trip to the store for another 5 gallons. The cost in deck maintenance is in the proper preparation and application, not in the money spent on materials. Inexpensive materials that last only a year are no bargain when compared with expensive materials that last a lot longer.

I have also found that premature stain or coating failures can almost always be traced back to the use (or, more accurately, misuse) of chlorine bleach to clean the wood before staining. Since wood is slightly acidic, stains are formulated to work with that in mind (pH of less than 7.0). But bleaches are highly alkaline, with a pH of about 12, which causes the pH of the wood surface to rise off the charts after cleaning unless the bleach has been neutralized. The result is that the stain is eaten alive from the inside and efflorescence (that white chalky stuff from suspended salts) blooms on the surface. Never use bleach. Never.

John Ricketson (from online comments)