IMHO buy a laptop...you can add an extra montor if you think you want or need a bigger screen...this also lets you do the dual display thing - allowing you to work on more than one thing at once...a desktop can do dual display too though...but just get a docking station for the laptop and plug the printer, mouse, keyboard, etc etc into the docking station then all you have is one cord to remove and you are mobile....

Headed to the inlaws for a tedious weekend?  Take the laptop.  Going to see the accountant?  Take the laptop if you want.  Personally i see no good reason to not allow yourself that flexibility.

Some of the flexibility that you get with a laptop will be unnecessary if you ever have plans to add an Ipad or Android tablet.