If you ate not going to war and already own a FS, take the Shield. You will notice the difference in width when carrying and concealing the smaller Shield, even aganist the smaller 9c.
I went through the same debate. Ended up with the shield because it gives me more options. You already have a large, get a small. The medium will not serve as much a purpose for you. My opinion...
I went through the same debate, but I didn't have a FS. I still choose the Shield, even though the 9c might have been a better compromise. I chose the shield because while it works around the house, it excels at CC. The price point was also an issue for me. I couldn't see spending the extra money to be able to carry a few extra rounds, especially since I'd be giving up conceal-ability.
I also have both, but if I had to pick only one I'd probably go with the Shield. The 9c feels better in my hands and I shoot a slightly tighter grouping at the range but only slightly. I doubt you would be unhappy with either. Try them both out and see which works best for you... or just do what the rest of us did and buy both
Put me in the 9c camp. I have both, and routinely carry the 9c (though I've been carrying my 45c the last couple days). I like the shield, and carry it occasionally as a pocket gun, but much prefer the 9c for IWB carry.