Be careful if you're thinking of Flying Blue (the Air France frequent flyer program). They did a major change last April in their points-earning scheme, and now if you fly discount fares, earning status is rather more difficult. Some fares don't earn miles, and some that do are at 25% of actual miles flown.

If you're planning on flying Business or full-fare economy it won't hurt as much, but if you're planning to fly on discount economy fares, flying Air France but crediting the miles to Delta might be better.

One other option you might consider is flying BA but crediting the miles to AA.