In the past 4 years, through work, I've had both a 13" and 15" MacBook Pro, the latest being the 13". I'm a software engineer so I'm not using the computer for video editing or anything like that. I can tell you that while I initially missed the larger screen (and quad-core CPU, I guess) when going to a 13", I've not looked back. I immediately loved the smaller form factor of the 13". For any meaningful work where I want lots of things displayed I don't find a 15" to be
that much better. I'll always go for using an external monitor for stuff like that if possible. Also, for the work I do, I really have no problems with the 13". I make the most of Spaces, fullscreen apps, etc.
In summary, the 13", in my opinion, is much less cumbersome and easier to use in different places (led on the sofa, travelling, etc.). I was due to buy my own this year and I was
really hoping we'd get quad-core CPUs in the 13". Now that we have that, I ordered a 13" i7 with 16 GB RAM and I can't wait to get it in a few days. To me, it's the best of both worlds. That said, I'm glad both models exist so everyone has something to fit their requirements.