Currently at work I and all of my co-workers all have dual Dell 30" 2560x1600 monitors. I agree that screen real-estate (resolution, not directly the physical size) makes a huge difference. I wouldn't go back to a single 1200p (never 1080p) setup ever again; I have dual 28" 1200p screens at home for the same reason (not the 1600p ones because of cost at home).
However, I am unsure of the 39" form factor for a single monitor; I think I'd rather have dual 30" monitors at lower res than a single 4k at 39". Though, the new 31.5" 4k screens from Dell/ASUS/etc would be a nice replacement for my 30" ones...