The comparison of the M&P 45 and the G21/21SF is a popular one at the moment, it seems, and for good reason.
In another forum's thread on this topic I posted the following info regarding likes & dislikes of the M&P 45:
Off the top of my head, as an armorer ...
Likes:
Replaceable grip & palm swell inserts
Beveled slide for easier holstering
Steel sub-chassis (to reinforce frame)
Coil pins (to secure L/B & Sear Housing to sub-chassis, distribute force)
Replaceable frame rails (w/o having to replace frame)
Steel Sear Housing Block
Heavy duty extractor (dimensions, height of hook & front shoulder)
Heavy duty slide rails
Self-centering frame rails (rocker rails) as wear occurs
Heavy duty slide dustcover (versus guide ring on Glock if dropped onto hard surface)
Easily grasped slide serrations
Steel sights
Trigger (revolver-like)
Stainless steel recoil spring assembly
Through-hardened stainless steel for slide & barrel
Teflon treated mag springs
Steel mag bodies
Option of mag safety which is simpler than 3rd gen design
Option of thumb safety
Reversible mag catch
Dislikes:
Solid extractor pins (would prefer roll pin like on the .45's)
Use of slave pin to align trigger bar & slide stop lever assembly (minor annoyance)
Small spring plate under rear sight (unlike larger one used in 3rd gen .45's)
Light effort regarding thumb safety engagement
Lack of plain stainless finish option on slide & barrel
Since I train for defensive shooting and not sporting venues, I shoot to trigger recovery and not trigger reset. The initial .3" rest-to-fire trigger pull and the subsequent approx .14" reset is not an issue for me (S&W specs).
Both of my M&P triggers lightened up and smoothed out considerably after shooting. My M&P 45 started out giving me averages between 8 1/2 - 9+ lbs NIB, on the heavy end of the expected 7 lbs +/- 2 lbs range for the .45's, but when I checked it after more than 2,500 rounds I was seeing averaged readings in the 5 1/2 - 6 lbs range (stock).
Now, regarding the G21SF, I'd post the following ...
Likes:
Better grip than the G21
Dislikes:
Trigger (typical forward sloping angle found on Glock, no curve)
No grip tang
Slide serrations (hard to grasp when wet or cold)
Narrow and thin guide ring on slide
Large dimension at bottom of grip
Plastic sights
Now, the trigger "feel" is pretty subjective. Being a long time revolver shooter, and having carried traditional double action service pistols since my agency transitioned away from revolvers, I've always liked a revolver-like curve to a trigger (and this is from a long time 1911 owner & shooter, too
). The steeply forward sloping angle of the Glock has never felt quite like a "good" trigger to me, although I've acclimated myself to them over the years. You deal with what you're given.
Grip angle and preference is fairly subjective, too. Maybe Glock will do something about the bottom heavy 'fat' grip on the Gen4 G21's. As it is, the M&P 45 offers a pretty good range of backstrap reach and palm swell dimensions on the M&P 45. The smallest of the inserts actually feels akin to a 1911 with a flat mainspring housing, from my perspective. Pretty handy.
Both pistols seem to be suitable for their purpose, and either ought to serve a prospective owner/user well enough as a duty-type/defensive full size .45 pistol.