I voted over the top.
It is visually more appealing.
I've had bad experiences with it the other way. If it is under, I find it is very difficult to get the paper out. This also depends on the ake of the toilet roll holder, expecially if it is sunk into the wall.
I once grabbed for a bit when it was 'under' and I banged my nails against the metal holder. My nail got so mangeled. There was blood. Well luckily by then I had torn off the roll had plenty of TP for my cause.
I also don't like it under b/c sometimes the tail ends up laying against the wall and that seems gross to me. I like my TP to touch nothing.
With that being said, the following goes against it. I'm lucky if the new TP roll makes it on to the roll. I've had too many roll away from me...I guess when that happens it is rolling from the under way.
That whole 'under' TP movement is out to get me.
So out of personal saftey I go for roll over.