The logical starting position would surely be to look at which airport in Washington is most conveniant and where most travel will be to and find the airline that best is going to meet your travel needs in the easiest manner

Once that has been done, then would be the best time to figure what programme will be best suited based on benefits and earnings to redemptions value

For someone frequently travelling in 1st and/or business class paid travel , the most valuable scheme may be different to someone travelling in economy

For domestic US travel, Washington is a United hub , so would likely be very good choice for that plus Star has decent options to Europe from there too

Other US carriers are , I suspect, much more likely to require connecting flights for domestic travel than UA will