I'm a fan of shoulder holsters.
Be reasonabley safe, secure, and comfortable with whichever mode of carry you choose for yourself.
Currently I carry only DAO firearms in my horizontal shoulder holsters. I limit my horizontal carry to DAO because I figure it's pretty hard to get a DAO firearm to go off without a substantial pull of the trigger, so carrying horizontally isn't much of an issue in my mind - as long as the gun's trigger isn't touched in my drawing it.
To me, handling and carrying are not the same thing.
Once, at a LE shotgun training course where the four golden rules were strictly enforced, I had my Remington 870 unloaded, slung, muzzle down, safety on and action open, with my hands nowhere near the trigger. While I was talking with some of my fellow trainees one of the range masters walked up to me and said "Do you know your muzzle is pointing at this officers foot?". I looked down and sure enough it was.
This event got me to thinking about how easy it is, without any intention, to "sweep" another person in the course of carrying a firearm. I started watching others. Heck, even LE instructors, with their keen sense of firearms safety, occasionally and unintentionally point a holstered, slung or encased firearm's muzzle at someone else or theirself at least some of the time, regardless of the holster, sling or case used.
I've often seen folks I consider very safe with firearms and well versed and practiced in the basic firearms safety rules, carrying a cased gun vertically as they moved about.
I've been in many places where a firearm was just sitting on a table, counter top or in a case and the muzzle may have been pointing at me or others; gun shops, homes, businesses and in the field.
In my decades of firearms use and my decades of following of firearms safety rules, I've come to believe that rule one (All firearms are always loaded, treat them that way) of the basic four is the one you have to practice all the time; and rules two through four apply while handling.
Rules two (Never point a firearm at anything you're not willing to destroy) and three (Keep your finger off the trigger, unless you're prepared for a shot) would be difficult, maybe even impossible, to religiously adhere to, unless you only handle your firearms alone, in an underground bunker lined with bullet absorbing materials. Rule four (Be sure of your target and what's beyond) clearly only applies while handling a firearm.
I'm not saying ignore any of the rules; I'm saying let's not become so wrapped around rule two we're afraid of firearms carried in reasonably secure and reasonably safe fashions. It is the handling of firearms that should be our utmost concern.