IMO the S&W M&P all the way....
Great feel, interchangeable grips for one person to next...
Both L andR side slide release....
Magazine release, interchangeable from L to R, though I would love to see a change going to the slide release on both sides

Its a great looking and reliable firearm, and has many benefits to provide 2 users comfort and reliability in 1 gun.

I have th 40 cal, have not shot the 9mm. But the 40 due to ergonomic design does not recoil as harsh as other 40s IMO. That may or may not be true with 9mm but I have to believe that the design would help with that to.

I own a Glock 26 9mm, and recently purchased an XD .45...
IMO the M&P is SOOOO much more comfortable.
Just my $.02
Good luck with your decision.
Jeff