Know this route well as I live SAT and worked a DC job for years. IAD-SAT is your best UA option if you value direct flights/shortest duration of flight. (Add in the time required to get to IAD and the hassle of negotiating the TSA and gate access process versus the ease of getting to and accessing gates at DCA) Right now there's only one direct nonstop flight per day leaving IAD 7PM and arriving SAT 9:39PM on an Embraer 175. Originating from DCA requires a connection (usually IAH) but it's a pretty easy connection and even if you have to contend with a delay on the DCA-IAH leg there are plenty of IAH-SAT flights most days.

My preference would be to take a midday DCA departure connecting in IAH. Many of the DCA-IAH flights are also serviced by "Barbie jets" but they tend to be the Embraer 175s which are actually pretty comfortable.