Now you ask - Which is better? Horizontal or vertical? Well, I don't think there is any difference in filtering, but when you change the filter, I think one is better than the other. I would say that I would rather have it horizontal because when you change the oil, it comes out of the thread end of the filter and down into the oil catch pan instead of all over the oil filter and down onto your hands.
Then why do I have mine vertical you ask! I installed it vertical without thinking about it. I may change that at a later time.
That info and $1.50 will get you a cup of coffee.