The horizontal won't do much. The vertical if positioned correctly can help. In the first few months just having it there enhances things - just the weight of the metal can be stimulating. After some time you kind of get used to it so you actually have to "handle" the jewelry like a toy to get the best benefit. Also you need to be with someone who can adjust and use a very gentle touch in the area since the person would be getting "extra assistance" to stimulate and if you are with someone who is unskilled, it can end up being too sensitive rather than pleasurable. If you get vertical clit hood in combination with something like a triangle piercing it can be fantastic. Find a good piercer and let her know you are looking for stimulation rather than decoration, if she tells you there is no difference blah blah, find another piercer. I would recommend Venus Modern Body Arts if you ever find yourself in New York City. Just beware anecdotally injuries have been reported with the clit hood+triangle combination, read up on them and discuss with your piercer the risks, if she doesn't understand what you are talking about, then find a different piercer. Also you probably want to choose a woman piercer and don't choose a piercer who acts like she knows more about your body than you do, because no one does, not even your doctor. All the best!
As I said, it's different and the way you'll experience will vary. As will the way you will get most stimulation from it - appart from personal reasons, anatomy plays a big part in this - it's not all black and white.
The triangle/horizontal clitoral hood combination has been known to be the most optimal combination to experience the most stimulation.. THOUGH, lets be real here, it's not something that attainble. Roughly 10% of all ladies are actually build for this piercing as it has to be safely placed underneath the actual clit. For most, doing a piercing like this will be either impossible or would involve huge risks as most piercers will not know how to asses their anatomy to suit a triangle, nor know the propper techniques. In adition, this is a much, much more intense piercing to undergo and will take a lot longer to heal. It's not at all that compareble to a clitoral hood piercing.
Yes, nobody will know your body better than you do. But by choosing to trust in a professional to do their job to the best possible end, you do actually have to trust them; nobody will know piercing, like a well trained and experienced bodypiercer is supposed to. I'd rather tell you to look for an artist that can balance both aspects, instead of either which!
And appart from it beeing flathering - sex doesn't say anything about an artists abilities; a male or female piercer.. It's all about personal preference.
We have this saying around here that would seem fitting, "schoenmaker blijf bij je leest"... (i'm having trouble translating, bu maybe other dutch'ies would care to help)
Either way, I'd wish you happy healing!