I spared no expense and got the California dust top. I didn't have a requirement for outdoor use because when its out of the garage...I'm driving it I've only had it for since late january and so far practically no dust has gotten through it. It's almost weird - even when my wife and I shake it out- doesn't seem like much dust comes out of it - almost like it repels dust. I had old cover a long time and was saturated with dust to the point where it was like not having a cover at all...and when we shook it out - dust clouds galore. I assume that someday this one will degrade as well....but so far I've been extremely impressed - and I should be for the cost! HTH
I ended up getting one from Carmotorsports. It is supposed to really block the dust. Won't provide much protection against impacts but I don't foresee any of those.
I had them put on the "Grand National" with the power 6 logo on the nose of the cover. Looks awesome.
I've got a Dustop cover from California Car cover and I have to agree, they work very well. Like Jooset said they almost seem to repel dust, and if your car stays in the garage you can't beat it.