depends on what you do, I use both, my laptop is next to my desktop when I´m home. Right now I´m waiting for the docking station, it´s hard to work on a 15 inch monitor when you´re used to 19 or bigger. If you get a badass notebook like a p4 3.0 with a 17inch monitor it could replace your desktop, but I think battery time sucks when you´re on the road and it´s bulky to carry too, you should think about it pretty well
