As others on here have and will tell you, a 15" MBP with its discrete GPU will perform significantly better for FCPX and other GPU-optimized apps. That said, you specifically asked about the 13" MBP vs the 13" Air. I'd go with the Air mostly for its higher-resolution screen (which also reflects a bit less glare). The 13" MBP has a claustrophobic 1280x800 screen while the Air has the same 1440x900 resolution as the base legacy 15" MBP. The 256GB SSD in the Air is also $100 less expensive than the optional 265GB SSD in the 13" MBP.
The only reason I'd go for the MBP is if you plan to predominantly use it with a second display and/or if you are a tinkerer and are interested in
swapping out the ODD for a second HDD or SSD. You can also swap in 3rd party RAM up to 16GB in the MBP, while the Air has its RAM soldered onto the motherboard and is maxxed at 8GB.
Get the Air, especially if you are going to be getting an iMac for most of your editing work.
Best,
Andy