If it's polycarbonate, it's a plain ol' MacBook, not a Pro. js.
As far as what works best on which computer, there's no definitive answer. I've seen people say they need a dedicated graphics card for CAD, and I've seen people say it works fine on the Air. You'll have to post more info about the computers!
Ok....Well, in my opinion, get the white one if you need processing power and future upgrades.
The mini is nice if you'll just use it for home or office.