The other stall. I’d never put myself between a horse and something solid in that small of a space.
I put up both butt bars when traveling, both to stabilize the divider and to keep the one in the empty stall from swinging and banging into either the mare or the padding on the wall. Forgot to do that once and wound up with a bunch of scrapes and bumps on her hocks!
I am less fanatical about the other chest bar, but I do tend to put it up–after she is secured in her stall. if you are nimble, you could leave it up and duck under it…I am both tall and NOT nimble anymore.