In your case, I'd go for the Macbook, the MBP is expensive for higher features you don't need.
Do you plan to keep the iMac 20"? If so, you can always go back to it for space-needy works. Although if it's a pre-Intel iMac, you might prefer the Macbook.
Regarding the screen, well, the Macbook's screen is not that small, although it has a smaller resolution than your iMac or the wide MBP... However, a big screen is comfortable, but if you want your notebook to be portable, you might want to keep it "small". I always think that the biggest versions of the MBP might be for people who need to impress their clients, or for people who don't really plan to move around a lot. My iBook G3 12" is crap for performances, but I like that I can just put it in my bag
EDIT: "I always think that the biggest versions of the MBP might be for people who need to impress their clients"
with big, powerful presentations or slide shows (not with the size of their wallet !)