So, which is the better *defensive* handgun, a S&W .357 mag fitted with
laser grips or a Glock auto?

Either.

I ahve my .357 loaded with .38's because I can do without the racket.
and at close range they both do the same damage.

i ahve my .44 loaded with .44 Smith's for the same reason.

If I can't do it with a Special, I'm not going to do it with a magnum
either.