Swiffer Duster all the way.
Not to be confused with the swiffer sweeper, the Swiffer Duster works just as well as the California duster. It’s dirt-cheap. Use it a couple of times and toss it out, since you can buy them in 5 and 10 packs. You can find them everywhere (no stores around here carry the California duster, so I have to mail order them).
Best of all, the swiffer duster is not as big and noticeable as the big red Cal duster, so you can discreetly dust your car off (especially that rear bumper) in the parking lot at work before putting the car cover on, without attracting the attention and ribbing of all your co-workers.
Seriously though, I actually use the Cal Duster and the Swiffer on daily basis. The Cal duster is larger, so you can dust the hood, roof, and trunk real fast. The smaller swiffer is great for all the small areas, curves, and vents. It gets right in-between the vent louvers and gets the dust right out. The swiffer is also easier to maneuver inside the car as well, to dust off the dash, console, doors, sills, and vents.
Just remember with all these dusters that they are just that “dusters.” Never apply any pressure to the car with them. Anything that doesn’t come off with a couple of feather light passes will need to be washed off, or you risk scratching the paint.