Almost the only reason to go with a shoulder holster is if you think you are more likely to need to draw from a seated position, for example if you do a lot of driving, or you're seated at a desk all day in an office where you have to deal with the public. The only other great reason I can think of is that you can get to a gun in a shoulder holster while wearing your overcoat buttoned, and you can't with a gun in a belt holster.
I like vertical shoulder holsters better than horizontal. The horizontal ones just don't conceal well. Not at all. They make a noticeable bulge on the left side of your jacket because gravity pulls the butt down and away from your body, and only the very shortest barrels don't print through the back of your jacket, especially when you lean forward or bend over. I have a Remington Model 51, which has to be a prime candidate for the world's flattest pistol, and I can make it disappear with either the Ed Buffaloe pancake holster I have, or the belly band I use when I want to wear it under a shirt. It will disappear under the lightest of t-shirts when worn this way. If I put it in the Galco Classic Lite horizontal shoulder rig I once bought (made for the Walther PP, but a perfect fit for the Remington), it doesn't really even conceal well under a heavy tweed sport coat. Just the way it distorts the shape and drape of the jacket is noticeable. With the belt holster, you can't see a thing. And I can't button the jacket over it; the gun just prints far too clearly if I do. And remember, this is with a very small, flat autoloader. Also, with the horizontal draw, you have to leave your top overcoat buttons undone if you want to be able to draw the pistol. The horizontal draw is more popular with shoulder rigs these days, because it's a little faster on the draw, but to my mind, the lack of concealability is a huge minus, and the speed advantage only comes if you leave your coat buttons undone -- and if I have to do that, I might as well stick with the belt holster.
But I do like a vertical shoulder holster to carry a compact autoloader as a back up in sometimes. I can put the Remington in an X-15 (size 1, for compact handguns), and it still has some of the characteristic shoulder holster's bulge, but less than the horizontal rigs do, because the butt doesn't swing down and away from the body like it does in a horizontal holster, especially when you move around or bend forward, and the muzzle doesn't print through like it does with a horizontal holster. And since the vertical rig's draw is more upward, I can wear my overcoat buttoned up against really cold weather.
But be warned, if you want concealability, belt holsters conceal much, much better. They're usually faster to draw from (unless you're sitting down), and most people find them more comfortable to wear all day than a shoulder holster (though a properly fitting shoulder rig is really no problem to wear all day). But the shoulder holster is more suited to certain, particular circumstances, and if those circumstances don't apply to you, you will almost certainly be better off with a strong side belt holster.
Oh, and I should mention the upside down vertical: they are fast to draw from, but again, you have to leave your coat unbuttoned. And you muzzle your own arm -- and your brachial artery -- every time you draw. No thank you!
