If I could only have one, it would definitely be a laptop...no question.
We have a Mac Pro in our house that serves a multitude of function (including being my wife's main work place at home)...it also supports a home entertainment network. These things would be difficult to do with a laptop as the Mac Pro has a ton of stuff connected to it at any given time (hard drives, accessories, camera equipment, etc). Having the extra USB inputs allows it to be a sort of hub. My Macbook Pro could not accomplish this, having only 2 USB inputs.
The positives of having the laptop though, for me, outweigh any of that because it is mobile. That is the defined function that keeps it my preference.