My reasoning for the east stairwell has been that the security manager who become trapped on Floor 7, used the west stairwell when he went up after the collapse of WTC 2, to look for personnel he feared might still be left on Floor 44. I would have expected him to maybe meet Jennings and Hess on their way down. No such meeting is mentioned any place I have looked; not that it need to be either. Next that they broke the window in the northeast corner, since that corner was closer to the stairwell. But of course, if they came down the west stairwell it would be natural to move as far away from the damaged southwest corner as possible. Another indication, is that Jennings in the BBC interview, describes how he moves over to the other side of the building finding it gone.
When I look at the floor schematics of Floor 6 and 7 on page 50 and 51 in NIST NCSTAR 1-9, I have speculated as you that the stair changed direction on Floor 6. If so my guess is the east stairwell, since the schematic indicates a rather large landing there. But as mentioned by someone earlier in the thread, they may have been so far down as floor 5 where the stairs ends in transfer corridors. That would also explain why Hess hit a wall in the dark, while likely attempting a 180 degrees turn, expecting to move down the next flight of stairs.
I have found no indication of damage to the stairwells. On page 300 in NIST NCSTAR 1-9, there is a description of damage assessment done by firefighters moving up the east stairwell to Floor 5, then moving over to the west stairwell and moving up to the 9th floor.
Regarding Jennings claiming that the landing gave way during the collapse of WTC 1, my take is that it only happened in his imagination. Quiet understandable when you in a state of fear in the dark, feel the floor/stair strongly shaking. He might very well have lost his balance.
So this is my take on the their choice of stairs, but I have seen no definitive information on this.
