I have both a 40c and a 9mm Shield. While the Shield is more comfortable to carry, mine, unfortunately, is "atypical". It has had 15 FTEs or FTFs in its first 2300 rounds. That is too unreliable for me to consider using it as a carry weapon. Until it can fire at least 500 consecutive rounds without a misfire, I will not carry it. So far, that hasn't happened, so I carry my 40c. It's really too bad, because when my Shield is working right, which is most of the time, it's a great-shooting gun.
My 40c has had only one misfire in 4400 rounds, so I have great confidence in it. Plus ten 40s are better than eight 9s, in my humble opinion. Despite being a few ounces heavier than the Shield, the 40c is not burdensome to carry. Most of the time, I carry it in a Desantis Nemesis pocket holster, but I also carry it in a 5.11 Tactical undershirt holster, or a Galco KingTuk IWB holster. Carrying in all of these is comfortable enough that I'm not sure I really need the Shield. Maybe if it becomes as reliable as the 40c, I can use it as a BUG.
Bottom line: If you're comfortable carrying your Shield, you'll likely become comfortable carrying an M&P compact.