I'll toss another one into your mix, since you're already considering an M&P. S&W has a new 'intermediary' between the low-end Sigma, and the M&P. It's called the SD, for Self-Defense. Closer to the Sigma in feel and price, but with the M&P in so far as looks and quality of trigger pull. I just got to fondle one yesterday, and I'm impressed.
On the plus side, it takes the lesser-cost Sigma mags, which are also nickled for corrosion resistance.
Plus, in comes with a front night-sight already dovetailed into place.