I have a calcarcover dustop cover for the Porsche - my requirements were like yours - protection indoors from dust while stored. The advantages of this material - very soft, flannel like surface touches the car - so minimal scratching - and not bulky. The downside of course is it won't do you any good in the rain or snow. I have used it outdoors on sunny days but wouldn't make a habit of it. I think it is a good way to go.
I also have a "NOAH" material cover for my M5 - it is OK for outdoor use. But it IS significantly bulkier and not as nice a material against the car.
If you're here at all you probably keep your finish in good shape. If you cover your car alone (which I do all the time) your cover WILL slide around on the paint - the paint is just so slippery that gravity will do its thing. So make sure whatever you get is a good fit, and that it will protect your paint. I always put my cars away clean, and I usually detail-spray them before covering to minimize the chances of dragging abrasive dust around with the cover.