A big part of my tavel is in the BOS to DCA route. That is served nonstop by US, and to a less extent DL, and an even lesser extent AA. While normally I like more connections for miles, work has gotten more demanding and I have had less time for flying extra segments. Probably 60% of my travel is on this route. I could go with UA, but IAD is too far from the city for real short trips. So it seems it is largely a choice between US and DL. One thing that keeps me with US is that you can upgrade any coach fare 3 days out as a Gold, but on DL the LUT fares can only be upgraded 12 hours before departure as Gold and 24 hours as Plat.