Well, I guess it depends on your situation.
Let me take my situation to explain:
I see myself as a non-traditional web developer during my own free time. I like to work at various locations (my desk, sofa, bed, outside, library, ...). But at my normal, regular work, I'm just a traditional developer who demanded a dual screen set-up at the job interview.
So you have to ask yourself, are you the more traditional type who does work at a desk? You're better off with a large screen area/desktop. If your desk doesn't matter for you and you're just going to work on a lot of various locations I'd say, go for the laptop. Just for the record, you will notice some people said they used laptops with external monitors. You should ask yourself if you will ever use the machine as a real laptop, because if you don't, you'll probably be off cheaper with a regular desktop due to better upgrade opportunities/longer life in general.