I haven't had a Shield 40 or a M&P 40c, but I have had both in 9mm...so while I can't address how they fire, I can tell you how they feel in my hand.
I have small hands...and on M&Ps, I always use the small back strap. The larger M&Ps (M&P 45 FS and Compact, M&P 40 FS, and M&P 9 FS) have always felt very comfortable in my hand...with the small back strap. The trigger reach was good for me too. The 9c with the small back strap felt OK front to back, and the trigger reach was good, but even with the extended base plate on the magazine the grip felt a little too short for me.
The Shield is the best feeling small pistol I have ever owned...with the 8 round magazine. The grip is too short with the 7 round magazine with the flat base plate. I love this gun for carry, and in fact it can work for home defense too. A lot of people depend on 5 or 6 shot revolvers, so 8 shots are certainly enough.