I agree, I worked with two 15"ers for a bit before I decided to go for the 19". Now I'm waiting to get a bigger desk and more money to get another 19"! Depending on what resolution you are comfortable using you can get 4 800x600 windows on one 1600x1200 display. On a 15" you probably use 1024x768, so do you want for the kind of work you're doing do you want 4 smaller windows or two slightly larger ones? The other benefit of a 19" is when you really do just need more space for one window, it's there in spades. it is also more comfortable to look at and will take up less desk space, although will be deeper no doubt. Not by as much as you'd think though, from a cunning design my Iiyama when placed in a corner gives me only about 1" less space in front ot it than my old CTX 15".
That size again: 19"
If you've got your money for nothing, who cares if the chicks are free!