I probably would, or maybe 7, to keep proportions similar.

Definitely 6'8".

Your door height to ceiling height ratio downstairs is 8 to 11 or 0.73 to 1. If you use 6'8" doors upstairs with the 9 ft ceilings, you will have a door height to ceiling height ration of 6.67 to 9 or 0.74 to 1. You can't get much closer than that.

7 ft tall doors would probably look okay but they'll be a bit taller in proportion to the 9 ft ceilings than the door height to ceiling height proportions downstairs.

Besides, 7 ft tall doors are a custom size at most companies which means that they'll cost you a whole lot more than otherwise identical 6'8" doors from the same company.