It depends more on what you are using the display space to do do. If you are chiefly concerned with organizing and editing Raw file takes of large numbers of exposures, or any other organizing function which involves having lots of image on-scree, a large screen area is functionally useful, and you get more area with two 20" screens than one 30".
If however seeing one image as critically as possible to do fine color correction, or to retouch you need to be close to the screen to see it most effectively so the real estate size is really not helpful, and if you're close to the screen some of the area of a 30" or two 20" displays is outside your field of best vision quality.
The "idea" of lots of screen real estate is often better than the actual experience. I know, been there and done that more than once. Unless you are a stock broker and have to view several spreadsheets at a time, its not much of an advantage to doing good photography processing.
Both a matter of personal work habits and the applications you use. I can't answer the question, but I can give examples of how I work here, and you can compare to your situation and draw whatever you can from them.
My main graphics workstation is used for image processing and 3-D content creation. It is set up with two 19" CRTs and I find it ideal. The left monitor is calibrated and is the work screen, with the right monitor used as the utility screen. Whether working in Photoshop or a 3-D application, I can have the workspace open full screen with no clutter. All the palettes are on the right monitor.
With Photoshop, Adobe Bridge also is running on the right screen, under the palettes until I click on it. If I am creating textures as I model in 3-D, Paint Shop Pro is open on the right screen under the palettes - of which there are many. Again a click brings it to the front.
It is horribly inefficient to use the main 3-D application on a single monitor, since most of the time is then spent wrangling palettes, opening, closing and moving them around. The right screen is also where I put the browser and directory management application. In essence, it is almost like working with two discrete computers that are intuitively connected to work together.
Video editing is done on a single monitor on a machine dedicated to audio and video capture and editing. If I did more, I would certainly consider a wide screen. It involves a horizontal time-line with the audio and video tracks all the way across the mid part of the window, a toolbar above and the monitor window, management and editing services across the lower part of the window. I have tried it on the dual monitor setup, and though the desktop is contiguous it is really not comfortable, working across two screens. A wide screen would be perfect in this case. Even better would be a wide screen for Vegas Video and a second monitor for SoundForge and anything else involved.
Music production is done on a single elderly 17" monitor, which is adequate, though the recording and editing of sound produced is done on the capture machine across the room from it and the rack of synthesizer modules. I suppose it would be possible to do production, capture and editing on a single machine with dual monitors, but so far, this seems the most natural and convenient.
So which is best? All of the above.