No idea why anyone would go for the 13" pro over the air now.
You gain aload of weight, a crappy screen and extra bulk for 30% faster CPU and upgradable ram/hd... woopdeedoo.
Woopdeedoo?
For starters you would save a significant amount from not having to build a custom spec air on the apple store if you ever wanted more than the stock builds.
Apple Store Custom Spec Prices:
128GB > 256GB SSD = £250.00
4GB RAM > 8GB RAM = £80.00
Alternatiavely buy a 13" MBP and then the following
OCZ Agility 3 240GB SSD @ £139.99
saving £110.01 and having a mechanical drive spare to do what you want with.
Corsair 8GB (2x4GB) DDR3 PC3-1066C9 1333MHz Memory @ £37.99
saving £42.01 and having a spare 2x2GB set of modules