Right off the bat I'll say my 13"
BUT I think the computer I
should be using most often is the 15".
After all it's only 1lb heavier, 1.5" wider and has a very very nice screen that seems much larger than the 13" screen. You can have two windows open, higher resolution and with the 15" you don't as often have the desire/need to hook up to a larger display. With the 13" there is times when you find the screen a little small and you're more inclined to want to hook up to an external monitor for some tasks or work from two computers, the other having a larger screen.
The fact is the 15" MBP can do it all and I think the best advice I could give myself would be to use the 15" MBP exclusively for a month and then go back and try the 13", I think I'd go back to the 15" real fast.
Still, the computer I like the most, practicality aside is my 13" MBP. It's just a little smaller / lighter and it's the computer I enjoy using the most, easy to manage, grab and go, use on your lap, plane, train etc. With the 15" while loving the screen you notice the extra bulk, lol.
Does it sound like I'm on the fence with laptop computers at all times? I am. Rather than thinking I have OCD I'd rather think a 15" MacBook Air would solve the problem
I envy my wife in this area, she runs part of our company and has access to and used the MacBook Air, 13" MBP and the 15" MBP ... I gave her my old Air she loved it ... then I let her try my 15" MBP for a days. Right away she ordered one and hasn't looked back. She uses the 15" and an iPad and couldn't be happier.
Me, well like I said a 15" MacBook Air (with 4gb ram, 256ssd, glass trackpad, and 6 hour battery) would be the cure all for me and I think a lot of others as well.