While I've spent some range time with the Shield 9, I've not yet tried a G43.
Having been through the armorer classes for both (G43/42 added to current Glock class, bu the Shield class is a separate module subsequent to the regular M&P class), I tend to think both are fine for their intended roles.
S&W says that the Shield is the number one selling pistol in the world
at the moment, and Glock said they G43 is
their number one selling model (out of their whole model line) for the last year. Bopth are selling as fast as they can be produced.
I know some other instructors who own/owned both, and a couple of them are what you might call Glock fans. One of them is rather ambivalent about his G43, and the other is regretting having sold his Shield to buy the G43.
I didn't get a Shield because I have a 3913, 3913TSW (late production) and a CS9. I didn't get a G43 for the same reason. Neither are what I'm looking for in a "pocketable" pistol, and I have a bunch of little single stack 9's, .40's & .45's.
If anyone is interested, Glock started using a special minus connector in the G43 about a year ago (not the same one used in their larger guns), replacing the original G43 connector, and this reduces the trigger pull weight down into the average weight of the larger Glocks.
Some folks seem prone to nit-pcik about the difference in mag capacity between the Shield (7/8) and the G43 (6), but it might be reasonable to bear in mind the intended role of this really diminutive single stack category.
My CS45 uses 6rd mags, my CS9 7rd mags and my 3913's 8rd mags. All of them hold
at least as many rounds as the 6-shot service revolvers I carried on-duty, and the Commander I carried off-duty.
Why 6-8 rounds wouldn't be sufficient for a really small off-duty/secondary weapon, or a really small private citizen's CCW choice, is a bit of a head-scratcher. Unless, of course, someone has in mind a fear of having to live out some fantasy shootout scenario inspired by the couple of John Wick movies.
Then again, these thoughts are from someone who has a plethora of pistols with 6, 7, 8, 9, 10 & 12rd magazines ... but who commonly carries a 5-shot snub or a LCP (6rd mag) as retirement weapons.
I'm not exactly a mag capacity junkie.
So ... both the Shield 9 and the G43 would seem to be fine choices for the smallish, lo-cap single stack pistol categories. Get one of each and compare them for a few years, yourselves.