Tiger, the snap cover is gasketed, I have never seen an in use cover that was. Most end up being a pretty loose fit that is terrible if the user isn't very careful to lock it down.
Maybe if you live in a place where the rain always falls in a gentle straight down way that isn't a problem but it rains sideways here as often as not.
Even your garden variety summer shower will be accompanied by 30-40 mph gusts.
