i had posted a similar question a couple of weeks ago in the car care section, you may want to look there for comments-
i had been surprised at the amount of water passing through my rather expensive cover that used the "block-it" technology - advice back was basically around how much you want to trade off breathability vs increased resistance to rain - for me, I'll be ordering one of the weathershield covers to block a bit more rain - at least until i can convince my neighbor to let me squeeze in a garage between our propeerties...
Cheers,
Kerry /ubbthreads/images/graemlins/canpatriot.GIF