Single monitor, less space needed.
Bigger screen space for single monitor apps (games)
Cheaper
Not as much real estate
Consumes less power.
Not everything supports widescreen, so you may end up with black bars
Dual monitor, more overall screen space (in terms of pixels you can see)
Bigger on screen stuff (if you have bad eyes)
Easier on a graphics card for games (lower resolution)
More expensive
Second monitor might not get used much
Need the desk space to place them