The macbook for me becomes, like mark said, a sketch book. like an artists has a sketch pad i have an easel. If if want to write outside this framework i put my lappy on a desk, attach a second monitor, all my usb midi devices and treat is as a tower.
im 100% sure mark can achieve the same results on both machines in terms of creation. but its the environment that really matters in terms of mixing down and monitoring. what sounded good on the road often needs serious tweaking when giving it the studio treatment.