I had the same question a month ago. I ended up purchasing both. At this point I have to admit I like the 9c much better than the Shield. I have medium size hands and for some reason I couldn't get comfortable with the Shield grip. The 9c feels much better and is much more stable for me. The 9c also feels more substantial, the slide is much easier to rack, I love the magazine with the extended base plate, you have 12 rounds, and a little longer sight radius. I really tried to get comfortable with the Shield, and will use it if I need to carry in the summer, but going from one to the other, is like moving from a high sprung mechanism to a serious, solid, weapon that knows what it is supposed to do.
For me the trade-off of dealing with the extra thickness is easy to make. When I pick up the 9c it's like I'm putting on a quality pair of leather gloves that fit perfectly - it just feels great. I also have a full size 9mm M&P and I like being able to use the same holsters, as well as being able to use the 17 round mags at the range. It's also nice that the sights are the same, so I can use the TruGlo sights I purchased for the FS on the 9c.
I'm glad I purchased both and I will probably cycle between them. But if I ever had some serious work to do I would feel a lot more comfortable with the 9c.
Definitely recommend shooting both before you decide.