South player, i don't know why, but south side always give you better position. Maybe it because when you in south, upper part of your screen don't closed by UI menu and it makes reaction on enemy units much easier.
A lot of people mention this, but it's a fallacy.
Imagine the area of your screen taken up by the UI is just gone, it doesn't exist and the UI that took it up is gone also. You have no UI getting in the way; it's all screen, just a smaller one.
In this case, both players have to move the camera the same distance to both be centred on the same unit (provided both players started the same distance away from the unit) and both players would have the same visibility around that unit and both players would see any units entering the visible area at the same time, be it from the bottom or top.
Now replace the UI and that viewable area of the screen is still the same size and still does not disadvantage either player. The only time the UI at the bottom of the screen has an effect is if the player tilts the camera, most don't, but if a player DOES tilt the camera is likely they will rotate it also to face the enemy.
Provided neither player rotates or tilts the camera, being north or south on a map has literally no quantifiable effect on player performance outside of preference and perception.