well...to my wife, dual setup is better for her.
She has two 15" LCD for her daily graphic, surfing, office..so on and so forth..(she doesn't play games, well, she admit woman can't play any game that require more than 2 button) Blahahaha.....
Cost-wise, a 18" LCD will cost me more than $1000, 3 month ago, wherea, two 15" LCD cost me bit less. 40% more viewing area, and cost less...why not?