I've run a gear diff for the last few months on medium to high grip tracks. Last night I switched to a ball diff. My observations...

The gear diff made the car less responsive on entry. On exit the car was loose at times.

The ball diff carried more corner speed and had better on power grip when turning.

My times improved by a half a second per lap do to the car being more consistant.

On super high grip I think I'd prefer the gear diff. But everywhere else a ball diff.