Since no one has commented about AA yet, I'll chime in with my .02 miles.

If you are concerned with upgrading low fares, it doesn't get any better than AA. Their upgrades (miles and upgrade certificates) are unrestricted by fare purchased, and that includes their weekend Net SAAvers. As far as I know, the only fares you can't upgrade on American are consolidator fares (booked in 'O', I believe). This applies to both domestic and international.

In terms of their route network out of DCA/IAD, it's not great. Travelling westbound you will connect at ORD or DFW, southbound at MIA and across the pond from JFK. But since connection times don't matter much to you that might not be an issue.

I will also add that I am very, very happy with AA's level of service. Sure, things happen, but they have always taken care of me. I'm Platinum now (50,000 miles flown a year) and will make the top tier, Executive Platinum (100,000) next week. All in all, a solid airline, in my opinion.

I hope that helps! Also check out the AA board for further information from folks far more knowledgeable than I.