I used to have a desktop, because the price got me better hardware for less money. Because I had my iPhone, e-mailing wasn't a problem. However, I realized that when I went on vacation that I missed having my computer with me--especially when I took pictures. I'd take all these pictures, but had nowhere to upload them to.
If you're a photographer or want to keep in touch when you're away on vacation, you should get the MBP. If these don't apply to you, go for the iMac.
Good luck.