Both,
Desktop for the multi monitor setup. Laptop for travel or coffee shop breaks.
I use the laptop along with the desktop for broker front end, and net surfing. Desktop is just charts.
When traveling, I take along a 19" monitor. Fits in suitcase easy. I just tape cardboard over the screen, and wrap in bubble wrap. I travel a lot.
BTW, Matrox will allow you to run 4 screens with a laptop, in high res. The laptop monitor + 3 external. You have to have a really good video card in the laptop though. The Nvidia GeForce with some HP's does the job. If you do this, get a 17" laptop, 3 19" LCD's, and 2 gigs RAM. If running XP, 1 gig will do. each external can run native res of a 19" (1280 X 1024).