i have used both. it's harder to do exhaust work wit a single as the arms can get in the way. i've read that if something goes wrong with it also you got to break out the jackhammer.
plus it doesn't take up any space above ground.
I would give my right eye to have one of these. Had them at a VW dealership in CT, I worked next door at the Audi shop on a single post, and had to borrow a lift to drop transmissions.