The biggest differences:
MBA has 256GB SSD (faster), while MBP has 750GB HD (larger) or add $400 for a 256GB SSD
MBA costs $200 more with the i7 and 8GB RAM upgrade
MBA has no optical drive; MBP does
MBA has no Ethernet, nor Firewire; MBP has both
MBA has 1440x900 display; MBP has only 1280x800
MBA is more portable
Only you can decide which of these is more important to you.