I bought the Classic (it was the only model at the time), but I doubt I'd buy the CO2 just because it's so wide. If you've got the right cab to mount it on, it'd be great, but it is very wide.
The Classic fit my needs, up to a point. I've modified my unit since, adding a topfire joystick and moving the spinner and 4-way joysticks around -- if you're curious take a look at my site. I've also written a review of the Classic unit I bought, which is posted on my site.
HeadRusch, I disagree that you'd save much more than about $100-150, at least if you bought all the parts new. I totaled it up, and a SlikStik classic could be made for about $275-300. They charge $435, so that's about $135-160 more. Considering the very professional construction of the unit, I think it's worth it.
Kevin