I bought my M&P 40c to carry, but it seemed a bit heavy, so a year ago, I bought a Shield 9 to carry. The Shield had reliability problems related to a batch of defective RSAs that were produced at the time, and it took me three months to get it sorted out. So I went back to carrying my perfectly reliable 40c. I got used to the weight, and I still carry it today. There's nothing wrong with the Shield. It is is now perfectly reliable, and is nearly as accurate as the 40c out to 15 yards. I took both to the range today and the Shield matched the 40c shot-for-shot. However, ten .40s trump eight 9s in my book, so the 40c gets carried and the Shield stays in reserve. However, I wouldn't hesitate to carry my Shield if I felt it was best to do so, so I practice with it regularly.
My advice: Rent them both at your local range, fire at least 100 rounds each before you decide. You can't really go wrong with either one.