I have used both (Que AC powered and VST bus powered) and, for the most part, they are fairly equal. The bus powered has the advantage of being more portable, generally smaller, and less cables to mess with. However, you do take a performance and price hit with the bus powered models. They are not as fast and I have not yet seen one with "burn-proof" technology. I don;t kow what this is, but ever since I got a Que fire CD-RW and used Toast 5 (both of which support burn-proof) I have not made a single coaster. This is nice.
With CD-RW's, as in all things, there are trade-offs. If you are going to use this in portable settings frequently, I would say the bus-powered is worth it. If it's going to be on the road only ocassionaly, go with AC powered.
Paco