If you're only going to be tracking 2 inputs at a time through your MBox 2 Pro,
almost any machine can handle that, but when you start overdubbing, mixing and adding effects to multiple tracks, that's where you need the processing power and the storage. The more RAM you have, the more tracks you can run.
The nice thing about having a larger display or two,
is that it's easier to monitor your primary DAW
application windows and all your plug-in windows.
Laptops are great in a professional environment, but if your running around like most musicians meeting lots of new people from all sorts of backgrounds, you have to keep in mind that a loaded MacBook Pro would be a high value theft target.
Without a primary workstation safe at home, you risk losing all your work and everything else you've accumulated on that system.
I'd go for the iMac and learn to take full advantage of your workstation, then
worry about portability once you get yourself up and running.
Your CPU ends up costing about one 3rd when you start adding plugin software, mics, stands, processors, amps and pre-amps, compressor/limiters, near field monitors, keyboards, and other studio accessories and that's just for a small project studio.