Forced to have one outside while the other enjoys a massage in my little garage, I've used for past 5 or 6 years 3 brands (2 locals and 1 us), all supposed to be made for outside. I can tell you one thing, none did the job as one would expect: keep the car dry. Most had that @breathing@ material you see now days on most of the models, supposed to stop water from penetrating while allow condense underneath to get out. In practice that material works fine for a short rain fall when new, after more than one hour in rain (something very common here) they all start to suck the water and car can't even dry in between rain falls. If the cover is more than one month old, it will no longer repeal and it sucks right away. Useless and may be worse than no cover at all.
This past winter, the first for my 73 outside, looks like I've found a combo that works. It's ugly as hell if in sight but car stays dry. Bought a new Max Tech outside cover, they are advertised as waterproof, that's a big lie like all others, but they do offer a perfect fit that can against hard winds, are relatively thick and affordable. I have on top of that, one of these big constructions plastic cover, with a size of 7 x 4m,. The excess allows no wind to blow rain or moisture underneath. As said it's ugly but this past winter that was really wet showed it worked fine. I would not recommend to do this unless you can like me undo the cover to take the car for a ride every week or 2 when weather is dry for a few hours, allowing the humidity trapped in between to escape. These Max tech covers would be great for inside storage.
I'm looking at some zip car solution instead of a cover for next year.. Much better for prolonged period of time I think. Time will tell.
If you find one that really keeps the car dry, please let us know in a few months from now.