Laptops, Definitely! You don't have all those wires getting in your way. And also, i was watching the Grammys on TV and sitting on the couch with the laptop watching the Grammy Cams and ATRL from the laptop at the same time. You can really multitask this way.
Laptops, by far.
I only use my iMac bc ArchiCAD software forces me to + i do all my Uni stuff there. Otherwise i'd never use it, i hate sitting behind a desk.