I have a couple questions that maybe could help and just for reference I have owned all 9mm Glock's except the 17L and 43, and I own a Shield 9 and did own an M&P9.
What do you intended to use the M&P for? Range, Carry, Home defense? and does the M&P9 provide you anything that the G17 does not? Since you already own the G17, unless you are dying for a reason to buy another gun, and you like the G17. I would stay Glock.
So here is my thoughts as I have confronted this conundrum myself. I prefer Glock over M&P for several reasons, with the exception of the Shield. I am actually currently wearing a Glock 30s in a N82 holster. My biggest gripes on the M&P9 is the trigger, good parts availability and holsters in that order. Yes, I could order an APEX trigger, and yes the trigger is better than previous models, but IMHO it sux. Wanna lighter trigger pull on your G17? A connector is $15, having to spend $500 plus for the gun and then $175 plus for a decent aftermarket trigger is not a good deal. Especially when there are other striker fired guns with actually good triggers stock in them.
I am picky about the sights on my gun, and changing them on Glocks is easy and you have a lot of options to find what you like best. The M&P on the other hand can be a chore to fit, given that not all M&P9's have the same sight cut dimensions in different production runs. Literally have seen set a Ameriglo's with set screw tightened slide out of the frame.
To me finding a really good holster is easier with Glock. Pretty much all holster makers are making Glock holsters, and you can find them on store shelves without ordering, that is not vice versa for M&P's. Also Glock magazines are plentiful, and you have cheap options with the magpul mags now.
Also and this is my personal preference. Glock simplicity, no yellow bar to flip down, no magazine disconnect safety, etc. The Glock is easier to work on.
S&W hit about as close to a gun home run as you can get with the Shield, and until they release an M&P that is same dimensions as the Glock 19. Im giving them a pass. But thats me.