Do a search and you will find 499 threads here addressing exactly that question
Sigh. Yes, we know. That doesn't invalidate the question asked. And your own opinion on the matter is what exactly, or do you have one?
Since they are very similar in terms of processing and graphics, and you wouldn't need more than 8GB of RAM being a casual user, it mainly comes down to hard drive space. A HDD on the MacBook Pro is very cheap per GB compared to the SSDs on the MacBook Air, which get very expensive very quickly if you want a decent amount of space. So how much space do you think you'll need?
Yes, a HDD is very cheap, and extraordinarily capacious, but a SSD is much faster, and a lot more stable (and less prone to breakdown). Needless to say, it is also very expensive, especially as the size of the SSD increases. Over time, it will come down in price, but is not doing so quickly enough.
A personal observation, here. My first Mac was the old 2008 MBP, which in common with all of the Windows computers I owned once upon a time (an Olivetti, a Toshiba, a Sony Vaio, all perfectly good computers) had the old standard HDD. I bought a MBA in September 2010, and would never willingly return to the days of the old HDD. While I am moaning about the occasional tardiness of my MBA, and muttering about upgrading, my brothers (one of whom inherited my Vaio) are awestruck and blinded by its speed.
So, yes, you need to decide how much space you want; also, decide whether you want a stable, speedy system.
It really depends on your lifestyle... If you plan on working on the go, then the Air is more valuable.
Being a former owner of a MBP 13, I have no regrets moving to the Air. If you wanted a Pro, then the 15 MBP is more valuable/bang for your buck.
Exactly. The bottom line is that the MBP 13" is certainly better value, but the MBA is faster, more portable (which matters to me, as I travel a lot), and - unless you plan to buy the retina model, which has a superb screen, the screen of the MBA is also better than that of the MBP.
Good luck with whichever you choose.