Dual. I'm so much more productive with dual monitors. More real estate. Two 1920x1080 monitors have more screen real estate than one 30" 2560x1600 monitor and cost less than half, depending on where you look
I have a VX2235wm Viewsonic 22" as my main but sometimes I have my netbook doing something too. It is kinda like having 2 monitors but one is way smaller then the other.