I once owned a Pre model 12 M&P Airweight, and currently own a Colt Cobra, the lightweight version of the Detective special.
Both are fine revolvers.
The model 12 is bigger in every dimention bieing a mid sized frame, while the D/S is a bit smaller. The D/S also has the advantage with it's short butt grip frame to accomadate grips that are nearly as compact as a J frame.
Sights are definitly better on the fixed sight Colts.
Trigger is subjective. I prefer the longer Colt action with the slight stack at the end during a slow pull (not noticable during a quick pull) , but the S&W trigger is good as well.
The model 12 probably will fit most hands better as it's a mid sized gun, but it's really on the large side for pocket carry. The D/S is better in that role.
Personally, I carry the Colt because it's nearly as compact as a J frame, and in a snubbie, pocket carry and compactness are most important for me, and having the sixth round is nice vs. a J frame.
For a midsize Revolver I carry a Colt too but with 4" of barrel.
One other thing to consider with the model 12 is that it's not a true K frame. It's a KA frame, which is a bit narrower, so K frame grips don't fit correctly. If aftermarket grips are something you desire, a model 12 may not be for you.