I've got a Galco Miami Classic that I carry my K9 in on occasion. I got it when my current pants were experiencing a little shrinkage, making it nearly impossible to button when wearing an IWB holster. Since I was damned if I was going to purchase some larger pants I got the shoulder holster as an interim measure. Since then I've lost some weight and my pants fit a little better.
One other consideration summers around here get a little too hot for wearing a cover garment all the time, even the satorial splendor of my light cotton Hawiian shirts, so I'm mostly carrying a 642 in a pocket holster. I imagine that when cooler weather comes along I'll be using the shoulder holster more often.
Tex