I downsized from a 15" simply for the portability. Occasionally I miss being able to comfortably open two windows at a time and still be functional, but that is rare. I feel like the 13" is a perfect size, just large enough to fit on my lap when I need it to, but not too large as to get in my way. The screen resolution is something else you need to consider. Right now, the 13" only comes in one size, whereas there is a "high-res" option for the 15". Their pixel densities for the stock screens are very similar, so text will appear the same size on either one.
If you plan to have the iMac as a primary machine, I think the 13 MBP will more than meet your needs and allow for more $ to upgrade the iMac (SSD, more ram, etc.)
Happy hunting! 🙂