Well... our brains already flip what our eyes see horizontally, so I'm sure it can do the same vertically.

actually your eyes would eventually flip themselves back over, but inverted, so either way you'd end up with a horizontal flip.

They already flip it vertically, actually, and it's been demonstrated that one's brain will start to see 'the right way up' after a while if one wears glasses that flip what you see

I would pick vertically. If our brains can compensate then it doesn't matter in the long run. In the short term however, vertically will be better. Why? Vertically is obviously wrong so it'll be harder to accidentally make mistakes, but horizontally actually looks like it could be correct, so you'd be more likely to walk into walls or in front of cars while you are adjusting.

actually your eyes would eventually flip themselves back over, but inverted, so either way you'd end up with a horizontal flip.