here's a comparison between the base 13" pro and 13" air, both around $1400
-pro has a faster cpu, 2.3ghz vs 1.7ghz
-both have same amount of ram (4gb)
-pro has upgradable ram
-both have same graphics chip
-air has a solid state drive, much faster than pro's 5400rpm drive
-air has a better screen, 1440x900 vs 1280x800 (even 11" air has a higher resolution than 13" pro lol)
-pro has a disc drive for now, upcoming pro will most likely lose it
-pro has legacy ports (ethernet, firewire etc.)
-air is lighter and thinner, more portable
purely performance wise, the macbook air wins, the ssd makes a much more noticable difference than the slight bump in cpu speed from the pro
if you still use old stuff like CDs, wired internet (lol srsly it's 2011) then you could just get an external drive and usb ethernet for the air, also firewire is long dead
if it were 6 months ago then i'd say get the macbook pro but since the airs have been upgraded with sandy bridge i see no point in getting a 13" macbook pro
ergo: at 13", get a macbook air