Glock 21 or M&P 45

[11mm]

If I could find one in 45acp, we wouldn't be having this conversation because I would already own one

Have you considered just waiting for what you really want then?
Of the 2 you've put forth though, the M&P.
The Glock has a better trigger but other than that the M&P is more ergonomic, as reliable in my experience and (IMO of course) comes with better sights and mitigates recoil better.

 

[burk]

Both good guns and I've owned both. Pluses and Minuses for both guns.

M & P 45
Positives
Fits smaller hands better
Extended 13 round mags are available
Crappy stock trigger still groups pretty good
Better stock sights than Glock
Available with 1911 style thumb safety.
Reliable and Accurate

Negatives
Mags are not as good of quality and fit and finish is poor (of magazines), the wraparounds on the extended mag fit is of laughable quality. And they cost more.
Crappy Trigger hurts quality feel of gun when shooting
Available with 1911 style thumb safety
Overall fit and finish is a notch below the Glock.
Accesscories cost more money because the aftermarket is not as Competitive.

Glock 21

Positives
Better stock trigger
Stock mags hold three more rounds and are cheaper
Stock mags fit three other guns as concealable options on same platorm.
Seems to have a little less recoil.
Aftermarket accessories are a much more competitive.
Gen 4 grip texture is great for non-slip in a variety of conditions.
It's a Glock, it works and hits what you point it at.

Negatives
This is gun for bigger hands, even the smallest grip option is on the big side
Stock sights (like all glocks) are poor.
Seems to sell for about $100 more new

 

[RustyShackelford]

Springfield Mass ....

Several gun industry reports & sources say Smith and Wesson finally gave in, re-engineering newer M&P series pistols. The new M&Ps from around 05/2015 on should have better triggers & re-designed parts.
These models should have a smoother, better trigger pull without after market custom work.

 

[Water-Man]

If I could find one in 45acp, we wouldn't be having this conversation because I would already own one

They were on Gunbroker.

 

[amprecon]

If possible, rent both at the same time, whichever one you shoot better with, get that one.

 

[bannockburn]

Between those two I would go with the M&P .45. The Glock 21 is just not a good fit ergonomically for my small hand.

 

[RoboDuck]

I have large hands and the Glock 21 grip fit me well. I bought a Glock 21 in 1992 , liked it so much I now have two .