For me there are other considerations beyond the obvious cash vs. raw miles. For instance, how many miles is my butt gonna be in a coach seat? In the case of JFK-SFO, 5,000 RT miles is a looooong way. The DFW-SEA trip is comparatively 3k RT miles - still a long way, but better.

I would also consider the time of travel and the time of year. In the summer time, when afternoon flight times have frequently meant to me the pain of sitting off gate for 2+ hours waiting for an FAA clearance because of storms in the flight path somewhere, there's no way I would risk that in coach.

Consider also that by not flying my preferred airline, I diminish my chance of retaining status that would keep my butt out of coach when it really matters, and it becomes a complicated decision.

I guess it would depend for me whether I had a new toy I wanted to buy with the money, and whether I had at least SOME interest in accumulating a few more miles on the cheaper airline.. otherwise, I go w/ my preferred airline. It's house money, after all.