With many laptops you can hook up an external screen and have 2 running at once. It gives you portability, but ease of use when you're at home.
I have 2 LCD monitors (19", 17") and it's almost necessary when working with Visual Studio. It becomes a joy when you're working with two items and you need to cut and paste as well. Or having SSMS and an editor, VS, etc. on the other side.
I like my desktop because for a lot lower price I can get 2GB of RAM and faster CPUs, but if you might need the portability (I have a separate laptop), then you might want to invest in just a laptop.