That's a tough choice, I have shot both , the Canik more than the M&P. The Canik is a very nice pistol, and you obviously have no bias against Turkish guns as you owned the Girsan. The Canik has a great trigger, good ergo's, cold hammer forged barrel, and seems to be a well built gun, but mags are expensive and I don't think it has anywhere near the aftermarket support the M&P has.
I've only shot the M&P twice, but it was ok, trigger wasn't on par with the Canik, but that can be fixed, and those who own them seem to love them. Will you have the opportunity to hold or shoot either or both before you purchase? That would be the best way for you to decide, but not always possible,lol. Good luck in your decision, I'm sure you will enjoy whichever you pick.