If you're buying solely for DC during sanding, the discussion of which unit is the most quiet is meaningless. Any of the 3 models will be significantly quieter than the sander.
All of the "top-end" vac-type dust collectors are very quiet...much quieter than a shop-vac or equivalent. Because of this, IMO, many other considerations are much more important than the noise level produced.
I have the Festool CT33, and it's been great. I got it as part of a package deal with a RO150, which saved me some $$$. If I were to be buying again though, I'd certainly consider the Fein or some of the other better units. They all would meet my needs, so I probably would make my decision based on price, cost of bags, and customer service.
Happy buying!
Where will you be when you get where you're going? -- Jerry Clower