Actually I haven't got to use the sanding wheel too often. For me the dremel is best for the cutting discs. It makes it effortless to chop parts off. I don't have a small saw, so normally I'd be forced to score the part over and over and over with a knife on both sides of the plastic. With the dremel it's muuuuuuch better.
I also have used the little metal grinding bit several times. I don't know what it's called.