Reason to buy the Macbook Pro (non-retina)
over the Macbook Air:
1. RAM can be upgraded up to 16 gig in Mackbook Pro
(actually purchase RAM from OWC or Newegg as it
it is much cheaper that Appple). The Air, what you get
is what you live with for ever.
2. You will be able to get off the shelf 2.5" HDD or SSD and
easily upgrade the MBP in the future. As, SSD densities
increase and price per gigabyte go down this is a very
attractive option to have. Also, if the drive dies, you can
easily replace it with the afore mentioned off the shelf drives.
Air, SSD is special form factor and is available only through
a very limited number of suppliers.
3. MBP better performance than Air.
4. MBP has internal CD/DVD drive if that is needed. No need to
carry an extra box around.
Reason to get Macbook Air:
Thin, light easy to carry with somewhat of a wow factor (although not
as much as it once was).
what about the resolution differences?
13" pro is 1280*800 and 13" air is 1440*900.
Since you'll be writing code, the higher resolution of the Pro may help you if you plan to have multiple windows or panes of code and you want to be able to spread out document windows and see them side by side. A lot of people prefer the Retina for reading text (like code). But if you think you will do most of your coding in one big window, then you may not need high resolution as much.
So think about your working habits. If you're a "one app fills the whole screen" guy, the Air screen is fine. If you're a multi-window worker like me, you might want the higher Pro resolution.
I like woodmeister50's point that if you want to get a Pro, the non-Retina is nice because it is so flexible and upgradable. That's why I own a non-Retina 15" Pro. But it's heavy. This is the problem...the 13" Pro is not as well specced as the 15" Pro so the 13" Pro is harder to justify against the 13" Air.