I currently have a 30" + 20" setup and a 27" at home. I find that I prefer the two screen setup even though 95% of my attention is on the primary screen. It's nice having "unimportant" stuff pushed off to the secondary screen (Skype, music, other chat stuff, etc.) and maintaining the feeling they're "there" physically but not in my face. It's mostly a psychological thing for me but that's still important.

I have found, and find, portrait mode to be maddening on all screens on which I've tried it but I'm fussy over visuals. Subpixel text rendering works best with the pixel shape in landscape and has somewhat less fidelity in portrait. As a UI developer, this might be even more annoying to you since what you see won't reflect what the majority of your audience is seeing (though it rarely does, to be fair).