definitly desktop - you cant beat being able to mod it to your specs whenever you want. plu with production these days you'll need endless amounts of firewire and usb ports (i think i have 10 usb on my pc as opposed to 4 on the lappy)

laptops are great for using as an fx machine thru your network or for gigging but id go for a desktop for full on production..

having said that theres plenty out there working just from lappy.. alex calver being one as far as i remember..