I like to use the sealant first and then H2O. I think the sealant would prove much more durable, but don't really have any evidence to back that up. Maybe a split window test is in order! Either way, I look at H2O as a way to replenish the GS and an easy way to seal the glass during the wash process. I think f you didn't have the GS it would probably be a decent option to just use H2O. But again durability wise, I would be willing to bet the GS goes a bit longer. Mine seems to last foooorrreeevvvvvveerrrr.