Both guns have a 12 round capacity depending on which M&Pc you're looking at. The Sig sub-compact is a little heavier than the M&Pc. If the M&P feels better in the hand, that would be my choice. Can you rent them to shoot before you buy? The Sig does offer the flexibility to change calibers to .357 or .40, all on the same frame.
The M&P is available with and without a thumb safety, depending on your preference and the model selected.
The other issue seems to be price, the M&PC msrp is $569, the Sig is $642 w/ Nite Sights. You would be getting a much bigger discount on the Sig if that matters to you.