I really think the 13" size is the sweet spot for me. When I'm at home, I hook my laptop up to a monitor, keyboard, and mouse, so I don't really need a big screen on the laptop itself. When I'm on the go, I'll take the portability of the 13 over a 15 anytime!
That being said, I have a 15" MBP because solid discrete graphics are important to me. I don't play many games but I do at least like to have that capability. I am interested to see what Apple does with the new 13s. If it continues to be the "low end" of the spectrum, power and features wise, I'll likely be moving to the new Sony Vaio Z. (I like OS X but I am not wed to either platform).
That being said, I have a 15" MBP because solid discrete graphics are important to me. I don't play many games but I do at least like to have that capability. I am interested to see what Apple does with the new 13s. If it continues to be the "low end" of the spectrum, power and features wise, I'll likely be moving to the new Sony Vaio Z. (I like OS X but I am not wed to either platform).