IMHO the Sigma is love it or hate it
for me, it was the latter
the SD40 is a sharp gun and feels the same in hand as the Sigma, but has a notably lighter trigger pull and the M&P sights aren't bad either!
the Sigma is a sharp gun (i love stainless slides), but the trigger was the deal breaker for me. just didn't want to force myself to like a gun. i didn't leave the SW family though!