Key thing with a shoulder holster is doing it right. Even a good shoulder rig will be uncomfortable and inefficient if poorly adjusted & fit. A poor shoulder holster will suck no matter what you do. The Miami Classic that SJustice mentions is a very good one, although old-school & far from cutting-edge. Not an insult at all, their even-older Jackass version is actually my favorite harness. A leather holster-body in a shoulder rig can be thick & bulky; I prefer Kydex, but that's largely because thinness (and anything else that helps concealability) is extremely important to me.
I've pretty much settled on the Jackass rig for the harness section of my rigs, which is basically just a softer, suede version of the miami classic. For the holster bodies I use kydex pull-thru's from www.survivalsheath.com. I have three set up this way; one for glock 19, one for glock 26, and one for kahr cm9's.
Biggest mistakes/problems when fitting & adjusting are that most people wear them too low. They can be more comfortable that way, but lose a lot of concealability and also flop around a lot. Adjusted high & tight under the pect, with muzzle diagonally up (which the miami classic style harnesses tend to), you can conceal a lot of gun under a jacket. Same way, you can also conceal a pretty-good gun (g26's & cm9's in my case) under a one-size-large, tucked-in dress shirt. Poorly adjusted, or poor rig to begin with, you just can't do that.
Upsides to a shoulder rig are that they don't require a stiff 'gunbelt' belt, if concealed properly, they can be harder to detect than a belt holster or pocket holster, and if you ever find yourself needing to use a stall in a public restroom, the gun remains holstered, undisturbed.
Downsides are that if in deeper concealment (ie, under a shirt), they tend to be slower to draw from; at least for me. They also (the diagonal & horizontal ones) also 'sweep' a whole lot of things throughout the day. The nice lady sitting behind you in church, your kid in the backseat of your car, etc. Not a danger with a good, protective holster, but it's a valid concern imo and bears consideration. This same sweeping means they can't be used in many training and competition settings, since they're pointing the wrong way (ie, at the officials, spectators, whoever) when you start to draw them.
As far as you personally with the G19's & G26's that you mention, at 5'10" & 150 you would likely have trouble accomplishing any under-the-shirt concealment. I'm 6'3" and 260-ish and I still have to be very conscious & intentional about it.