So, what can you tell me about them? The pros, cons, ect.
The rig that I have in mind is the Miami Classic by Galco, and the weapon that I'd be using is a Glock 27.
Thanks for the tips and advice. 
From the standpoint of concealment, the shoulder holster is pretty good. The key is keeping the holster snug to the body, so it does not flop around as you move. I believe the Classic does have a strap that fastens to the pants belt.
A shoulder holster works best with an automatic, the flat gun conceals better than a revolver.
Some holsters hold the gun vertically, others horizontally. The vertical holster is more secure but forces an awkward draw. The horizontal holster is the faster draw, but needs a safety strap to secure the gun.
The fit of the shoulder straps to your body is critical. Too short, and the holster will bite into your shoulders. Too loose and the holster will shift around on your shoulders.
Balance is important as well. I believe the classic has options to add a mag carrier and cuffs to the rig. Go with the mag carrier (at least). The two loaded mags will come close to balancing the weight of the gun.
If you are a large man (particularly through the chest), you will find the shoulder rig difficult to use. Fitting is a problem and sometimes even reaching for the weapon is a stretch for the shooting arm. Actually, women, with narrower chests, find it easier to draw with a shoulder holster.
The one big draw back to a shoulder holster is that you can not take off your jacket or coat if you want the gun to stay concealed.
During my stint as a police detective, I carried my Colt 1911 in a Bianchi X15 holster. This was a vertical draw holster. The 1911 was simply too large for horizontal carry (poked out beyond the body). I was doing a lot of weight training in those days, so my shoulders and chest were quite broad. Drawing the 1911 was very awkward. I finally switched to a strong side belt carry.
The downside to the shoulder holster is the coat you are wearing. You can not button or zip up the coat. Doing so means you can not get at the gun quickly. I once had to draw mine and actually caught the lining of my suit jacket on the hammer of my gun. You have to practice getting your coat out of the way of the draw. More than one cop has shot a hole in his coat/jacket. The draw with a shoulder holster is ALWAYS slower than the draw from a strong side holster. Simply because the gun has to come from the far side of your body all the way around to the strong side before you can shoot.
I can't remember if your Glock 27 is a full size or compact gun. But, either way, have you considered an Inside the Waistband holster? Just as concealable, but faster and more comfortable.
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