Based on my own personal experiences, the Sigs are worth the extra money. Not sure how much more, but easily +$200 or so. I love my M&Ps as well, but if I had to give up the M&Ps or the Sigs, the M&Ps would go without question.
The Sig's feel better in the hand, have been more accurate for me, and while I haven't put thousands of rounds through the M&P yet, I have with my Sigs and never had a single issue. 100% dead reliable, year after year after year. I hope, and expect, I will have similar performance with the M&Ps. And if I do, I'd still take the Sig over the M&P even at a premium price.
I can't comment on the SR9 as I have no first hand experience with them.