One large for sure. The reasons for that would be the same reasons that I'd prefer a single graphics card over an SLI or Crossfire setup.
- Less monitors = less hassle, less space, less heat, less power
- Dual monitors are painful to set up (the last time I tried on Vista anyway, I'm on Win 7 now and haven't tried it)
- More organized multitasking by having everything on the same giant monitor
- If you ever want to upgrade, you only have to get rid of one monitor instead of two
- 30" monitors are beasts. Two 24s can't compare.
I had a 20" monitor and wanted an upgrade. I thought about multimonitor so I borrowed my friend's 20" to see what multimonitor would be like and the end result was pretty meh, so I ordered a single 24" and I'm much happier with it.