I have both a MBA 13 and a Thinkpad 12.5 (with SSD) at home, both with 4GB ram, and they perform fine. The Thinkpad's trackpad really sucks though, it's horrible... it's too small, jumpy, very often my taps move the cursor away instead of registering as a click, and 2-finger scrolling lags or doesn't always register. The MBA's trackpad is waaay better, however I prefer the Thinkpad's keyboard.
For portability the MBA wins, since it's much thinner and therefore easier to put in a backpack.
Regarding apple laptops, I must say the Air 13 has the best screen for me... The Pro's screen is too reflective, while the Air 13's has an antireflective coating sort of like eye glasses do, toning down reflections to a purple color that isn't bothersome. For some reason the Air 11 isn't as good in toning down reflections as the Air 13 is.
The Thinkpad takes a different route (matte screen) to beat reflections even more effectively.
Basically you need to consider your preferences in OS, screen, keyboard, pointing device, performance, portability, and last but not least warranty.
I'm seriously disappointed by the thinkpad's trackpad however, it's worse than other windows laptop's I've tried.