JFK-HKG F v J
On JFK-YVR-HKG you stay on the plane. On the reverse, you have to get off and wait in the sterile, walled-off boarding area.
Not to bring up a sore subject for some - but if redeeming AA miles, the J to F difference is small enough that it would have to be a big hurdle on the "preferred date" issue to "waste" the miles on J. For other schemes where it's a 50% difference, that hurdle may be less. In any event, nobody here can gauge what "preferred" means for you.
And of course there's always the possibility an F award seat opens on your preferred date.