When I bought my 40C there was no M&P45. No doubt the M&P45 is a very nice gun, but with only 10+1 capacity, it doesn't interest me. I have plenty of 45s in all shapes and sizes and if I'm going to have the size and weight of a full size 45, I simply want more than 10+1 capacity. M&P buyers have a very clear preference, statistically, for the M&P40 over the M&P45.
40 offers relatively comparable ballistic performance and much larger capacity. While I like the push of 45 more than the snap of 40, but am equally comfortable shooting both of them.