Carried both for duty. If nothing goes wrong then both are great. If you are picking one up and want to do anything.... like night sights, a trigger upgrade or similar then get a Glock.
I say this because the rear sight on an M&P holds parts in place(?) while the Glock's cheap plastic just needs to be removed. You need a special tool (a small piece of plastic) to reinstall the trigger while the Glock requires you to move around the slide lock lever until it lines up. The front sight is put on by the Incredible Hulk when he is very, very angry which makes you very angry when you are beating on it (maybe a little) with a hammer and a brass punch while the Glock just unscrews. The takedown lever retaining wire is a separate $10 (minimum, not including shipping) part while the the part that does the same thing on a Glock cannot pop out and go missing making you search for hours for a magnet that might (won't) find the oddly bent staple gun staple causing you to buy one off of eBay.
Also:
- There is much more aftermarket support for Glocks
- Mags are cheaper and more plentiful. It is easier
- Most pistol caliber carbines offer "Glock" magwells, even those from Ruger and S&W
- Glocks are easier to work on (as mentioned above)
- Normally when you swap out the trigger on an M&P you end up with a "Glock style" trigger
- Customer service from Glock is (for me) so much better than Smith's
As far as personal taste goes.... pick CZ.