On the OP question, when using Sirius through the steering wheel buttons, clicking on the RH button brings up the channel list, so you can select a new station.
There is a nifty "back" button on the RH side that then brings the display back to the song being played. If you switch the LH and RH scroll wheels, the channel switch button and "back" button would be on separate sides of the wheel.
Don't know about you, but I'd find that pretty distracting while I'm driving. I have enough trouble getting one hand to do the correct thing!