I had the standard MC black cover and found it to be the best fitting cover I ever had. That was the good. The bad was that it was the worst cover I ever owned from a protection perspective. It left black marks on the seats and acted as if it wasn't there when it rained. This is all documented in multiple posts on this site. I tried using 303 waterproofing twice to see if I could seal it up with no luck. I finally ditched it for a Sunbrella cover from skiboatcovers.com and couldn't be happier. My dealer recommends this company/cover as an upgrade option to their customers from the MC cover.
As to fully waterproof materials.... If you go that route you have to install vents to keep the air moving. Quite some time ago I saw the results of a unvented vinyl cover on a boat. Anything wood in the boat was completely rotted in the course of just two years. IMO paying a couple hundred dollars more for Sunbrella is cheap insurance for a cover that should last you 7 - 10 years. The last Sunbrella cover I had from skiboatcovers.com was 6 years old when I sold the boat and was showing very little wear at the time. The moral of the story is that if you plan to keep the boat spend the money on the best material and cover you can possibly afford.