To be picky, whether or not you want/need the bigger screen with you all the time. I have only a MacBook and I use it with a 20" lcd at home, which is where I do the majority of my work. (I also have more than one desktop, so if the laptop is going to be a sole machine, yet another thing to take into consideration.)
I couldn't justify the additional cost, size/weight and fragility of a MBP over a regular MB, having owned one previously. My productivity doesn't suffer with Spaces and Expose on the 13" widescreen (although this is personal preference), and the external display is a cherry on top of the cake.
Of course, you could also do the same thing with a MBP, but assuming that budget may be a limiting factor and all..
I couldn't justify the additional cost, size/weight and fragility of a MBP over a regular MB, having owned one previously. My productivity doesn't suffer with Spaces and Expose on the 13" widescreen (although this is personal preference), and the external display is a cherry on top of the cake.
Of course, you could also do the same thing with a MBP, but assuming that budget may be a limiting factor and all..