Most people I know who’ve never used dual screen ask me why I don’t invest in one big monitor instead. There’s a big difference between having multiple monitors and having just one, no matter the size. Though which is better really depends on what you’re using the computer for, what OS/window manager you’re using, and how you use your particular setup.
For programming, and the way I use my desktop (gnome+compiz), having multiple screens is much more productive than one big screen because I hate minimizing and dragging windows. If I’m coding in rails for example, I can have my editor maximized in one screen, and a reference window or a `tail -f file.log` maximized in the other screen. With a big monitor I would need to manually resize each window or depend on my window managers cascade feature, which requires some manual resizing as well.
For coding or writing, a killer setup would be having two or three widescreen displays setup vertically side-by-side. I noticed I only look at 3/4th of the screen real estate when I’m coding, and when I hack or write, I generally prefer having one long narrow window (off topic, but I always exceed 80 chars per line). Widescreen wasn’t made for coding anyway.
When it comes to (photo) editing, whether you’re using 1 or 2 monitors, widescreen is a must. Many people complain that Gimp doesn’t support MDI (Multiple Document Interface: Having multiple windows in a container, like Photoshop or AOL), but I noticed that Photoshop on Mac doesn’t have MDI either, and I finally began Learning to Love the Gimp Interface. I can edit a picture full screen in one monitor, while I have my entire toolset open on another screen. I can also do all my editing in one screen and have tutorials or references on another display.
Taskbars on each screen are also separated, so that apps open in one desktop don’t show up on the taskbar of another desktop (tweakable). This simplifies the taskbar and makes it quicker to find what you have minimized. No more 256 pixel thick taskbars.
I used to think having a split/gap between screens was annoying, but it naturally segregates the workspace. I can banish windows I don’t care about to ‘the other’ screen and focus without distractions on my main desktop. It’s sorta like the ADHD plugin for compiz. I can also switch my attention to make which ever monitor I want to be my “main display.” This brings us to another huge advantage a multi-monitor setup has over a huge single display.
You can physically move each screen around independently, which is great when you’re working in teams (i.e., you wanna demo something) or you just have people over. Just last night I had a basketball game on my 2nd display tilted so my bro can watch (neither of us have cable) while I did my thing in ‘my own’ desktop.
With Devilspie, you can probably configure yourself a pretty nice setup for a big monitor, but if you don’t plan on doing much hacking, dual screens are naturally more productive in OS X and Linux. I noticed Windows XP has problems with dual screen, i.e., the taskbar doesn’t go across both monitors, and the cursor moves off screen and “breaks” into other desktops while you’re in a game. Very annoying, but Vista or Windows 7 might not exhibit these problems, and there are workarounds in XP.