Ill second the Smart Carry. You can easily carry pretty much anything, and you dont even need a shirt.
They are just about as fast to access as an IWB type holster, even while sitting. Very comfortable to wear, all day too. I have a couple, and they are my most versatile holsters, and Ill never be without one.
Shoulder holsters are about the worst. They are the easiest to spot, and thats under a coat, under a shirt would be even easier. They are not all that comfortable, especially towards the end of the day, and feel like a ton on the top of your shoulders and around your neck, and thats just with a gun in one. Add a reload or two, and its even worse.
If it absolutely has to be, a Galco "Executive", or a Ken Null "SMZ" would be my choice. They are very minimal holsters, and can work under the right cover garment, are reasonably comfortable for day long wear, but I still wouldnt think a shirt would work. Your also limited in the size of gun you carry with them.
You'd be better off with something like a Kramer Confidant undershirt holster. They do work, and very well, and are fairly easy to access. Just leave a button under your tie open, and its even quicker.
Prior to the Smart Carry's, I used one in just the capacity youre considering, and it worked out very well. Given the choice now though, it would be the Smart Carry without hesitation.
I have a couple of tuckable IWB belt holsters, but dont use them that way. The loops/attachment points still show, and your shirts just tend to look messy. Other than that, worn appendix, under a tee shirt in the summer, fleece or sweatshirt in the winter, they are my favorite way to carry/conceal a full sized gun, and the quickest to get at. IWB is very comfortable once you get used to it, and very stable. If the gun is moving around, and you have to keep adjusting things, youre doing something wrong.
With all but the Smart Carry's, if you wear a fitted shirt, your going to have to change them out for something fuller in cut. Just something to keep in mind.