For myself, I'm considering open carry with a shoulder holster, as it seems the most out of the way of moving parts, and I definitely prefer cross-draw. Anyone have any comments, experiences or reasons why not?
For years I carried my Colt Peacekeeper with a 6" barrel in a Bianchi x15 rig. Just recently I tried an Uncle Mikes sidekick shoulder holster, also a vertical holster, and bought it. The Uncle Mikes, and other rigs that have a tie down on the side opposite of the gun, are much more comfortable and secure. You can also use that tie down to mount spare mags or speedloaders. With the tie downs, the shoulder rigs are the most comfortable carry rigs going IMHO. The Uncle Mikes rig is almost infinitely adjustable---even the retention strap is adjustable for a tight fit. The snap buttons are the type where you have to put the snap on the button, twist and push to lock it on which adds security.
As for retention, the only way that someone is going to get the gun is to come at you from the front. They can't rip the holster off of you with the tie downs. The gun butt faces forward so there's no way someone can get ahold of the gun from the rear. The gun is tucked in close to your body and from the side, someone has to go through your arm to get the gun. It's as secure a carry as you're going to find.
I will say that one thing that you will also want is a good gunbelt. Don't buy cheap, go and get a Galco or a Wilderness nylon belt as these heavy belts provide a good foundation to anchor the ties too.
I would strongly discourage carrying revolvers in a horizontal holster as if the revolver is accidentallycocked, there is no additional safety (unlike a semi-auto) and the gun will be pointing up. I also dislike the horizontal holsters as anytime you're behind someone with these, their gun barrel is pointing at you!!