For equal cost, why would you select the MacBook Air over the MacBook Pro?
The primary cons for the MacBook Pro are:


heavier and more bulky compared to the Air
noticeably slower HDD versus SSD at the same price point
only has a 1280x800 screen resolution


Note: but is confirmed to be a better quality display than the one in the Air, with a more accurate color gamut.

Things that sold me on the MacBook Pro over the 13" MacBook Air: the MacBook Pro is more future-proof and upgradeable and offers more value for the money in my opinion. 


Primarily Upgradeable RAM, the Pro Comes with 4 GB and so does the MacBook Air, but the key point is that I can just order it with the 4 GB for the Pro, knowing that I will be able to upgrade to 8 GB or even 16 GB at market rate for RAM whenever I want to. I cannot do that with an Air, you either order the 4 GB and shorten the usable life of your computer or pay upfront to upgrade to at most 8 GB at a premium price (thats shoddy and I would like to avoid that).
Secondly the MacBook Pro as of Mid-2012 version still has a standard form factor drive bay where you can purchase and install 3rd party 2.5" SATA Laptop Drives of your choice in SSD or standard hard disk drives. 


And then the bonus features in order of importance to me, all of which without the need for external dongles and adapters:


Gigabit Ethernet 
Firewire 800 port
SuperDrive (optical drive)
Standard Audio headphone jack that can do both output or input over analog or digital cables
Hasn't moved to the new MagSafe 2 adapter format


So the flexibility and value of the MacBook Pro sold me on it, its really sad to note that all MacBooks will probably be Airs (like the MacBook Pro Retina) soon enough and this value option will be nothing but a thing of the past, so enjoy it while it lasts.