if it was down to choice, i would personally just save up a little bit more money on a $1300 budget to a $1600 one, and upgrade to a 13", with the i7 and the 8GB of RAM. enough screen space, still ultra portable, you will find it easier to see your FCP creations on a 13" screen, against a smaller 11" macbook air.
like i said in an earlier post, 11" MBAs are for those looking at a secondary mac to accompany thier first one, and the 13" macbook air is for those who need a primary mac, and nothing else, like the OP stated.