I have a 15" rMB and a rMBP and recently bought a MBA for my son, basically for schoolwork. Having used and using all three I cannot choose between them, as they each have their strengths and weaknesses. The screen of the MBA is totally unacceptable for me, and I mean really, really unacceptable. And the way it huffs and puffs when it has to work hard is annoying to me. But it is a very nice allround computer with plenty of ports and performance and it is light. My MBP on the other hand is waaaaay to heavy and big, and battery life is bit too short once it has to do something difficult. But the speed and the ports make up for it.
And than there is the MB. I bought my MB as a iPad successor, I just could not get used to IOS. The MB for me is THE perfect alternative to an iPad, with a decent processor, decent storage and a real keyboard, all in a incredible small package. As for performance, I have never understand where people notice the 'lag', as I have never ever noticed it. And even when working hard, it stays silent! But perfect it is not. Two USB ports would have been very nice, and the keyboard, while better than the keyboard of the iPad, feels very hard. Especially compared to the MBP. And the screen is a bit too small. Etc.
My perfect next computer: a 13" MB, with the battery life and price of a MA, the performance and ports of a MBP, and the silence and low weight of a MB. Won't happen though!