It all comes down to raw math I guess. Airbus A320 has only 12 seats total in First, and crappy F seats too IMO, and 4 of those vanish if equipment change to A319 takes place so that would be the LEAST desireable .... for ME. 757 has 24 First seats, so a no brainer if there is a choice.

Remember too that IAD and SFO are both major hubs, so more 1Ks on that route than most, hence heaps of 1Ks always ahead of you for 500 mile window AND DM list and on PA waitlist ahead of you before the day anyway most likely.

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~ Glen ~