Briefly, the MBA is a splendid machine, lovely screen, sharp and swift start-up (thanks to its SSD drive) and ultra portable. I have a 13" MBA.
Now, the plusses of the MBA are portability, nice screen and swift responses. it is a perfect machine for someone like me who 1) travels a lot and for whom portability matters & 2) is not a gamer.
There are minuses to the MBA: the capacity is less than that of the MBP (unless you are willing to pay stratospheric sums of money); the power is less, too. If you do a lot of gaming, or video-editing, or want to store many, many movies, then, I suggest that you try the MBP.
I have had both and both were excellent machines but served somewhat different functions. Someday, in the future, I will probably treat myself to a 15" rMBP with an obscenely large SSD drive; however, that day is still a long way off. For now, and also, for always, I will - in addition - always have some version of the MBA because I love the form factor and value the portability as I travel a lot.
You have to ask yourself what you really want, and (even more importantly) what you really need (what you cannot do without) from a computer. That, in turn, will tell you what machine to buy.
