Can't decide between a G43 and the G43x. Would the G43 work as good with a mag extension to lengthen the grip? Can it take the new S15 mags?perhaps with a grip adapter/filler? If I could have a gun I could switch from deep concealment to full grip backpack gun that would be great.

Would the G43 work as good with a mag extension to lengthen the grip?  Reliability has been the main concern of extensions, but many seem to work.
Can it take the new S15 mags?  No they cannot.

Body type matters a lot.

I have a Glock 43 and a Glock 48.   You can put the slide from a 43 onto a Glock 48 frame and have a 43X.   One advantage of the G-43 is it is small enough to be carried in an ankle holster and easier to conceal compared to a 43X.    My hand is small enough I carry the flat base pad magazine in the G43  and two spare magazines in a pocket.  I use Peace +1 mag extensions on my spare G-43 mags.  The Taran Tactical extensions are pricey but add enough length to the grip to be close to the grip length of a 43X.

I think your physical build and your intended use can determine whether the 43 or 43X will work best.   I carry the Glock 43 the most and I have the option of 43, 43X and 48.   I think the 43X is a little easier to shoot than a 43 plus you have 10+1 or with S15 mags 15+1 with the 43X.   I feel well armed with the Glock 43 with 6+1 and two 7 round spare mags.   I routinely carry twenty one rounds whether Glock 43, Glock 48 10+1 and one spare 10 round mag or Glock 27, 10+1 plus one 10 round spare mag.

Taran Tactical offer a plus 2 or a plus 3 magazine extension.   The +3 extension would give 9+1 capacity and would be very close to the grip length of a Glock 43x.

https://tarantacticalinnovations.com/base-pad-for-glock-43-9mm-oem-magazines/

I carried a G43 with a +2 since pretty much when they hit shelves.  It works fine.  Had zero issues with it, other than the capacity.
Then glock pulled their heads out of each others asses and made the G43x.  Its the same size as the G43 with a +2 extension, but holds 25% more bullets.  Get you the shield mags and you hold 87.5% more bullets.  Going from 8+1 to 15+1 in the same size package.  I even use my same holsters from my G43 on my G43x.  The back of the grip is also nicer profile.  G43 has a more pronounced hump compared to the G43X.

Here's my 2 retired CCW guns along side the current CCW (G43X).
G26 with a 15 round G19 mag and a xgrip
G43 with a +2
G43X with 10+1 (and not in the photo is my 15 and 20 round shield mags)

I have both and like both. If I could have only one it would be the 43. I have a TTI +2 on my spare mag and I’ve never had a malfunction. Pearce +0 on the carry mag.

I have a G43, G43X and G48. I love them all......  G43 for easy concealment. G43X during winter when concealment is easier due to winter clothes. G48 usually for hiking. If I had to have one for concealment.... G43 !

I think Glock has really dropped the ball with their lack of magazine capacity, this coming from a Glock fan. I picked up my Glock 43 several years ago.
It shoots great but I never liked the small mag capacity. I don't trust third party mags or extensions.

The shield 15 round mags changes things for the 43x but long term reliability is still to be determined.
I would look at the Sig p365, p365xl, or sa hellcat. From micro compact to magazines that allow a full grip.
I have 11 rounds in a gun that is the same size as the 43 and with the 13 round magazine it's shorter than a glock 19

The 43X conceals just fine. It's incredibly easy to conceal and carry, plus it has the enormous added bonus of having a grip that actually fits my hand (unlike the 43) and it holds more ammo.

I'm going to trade off a grip that actually fits my hand and higher capacity for a gun with a marginally smaller grip, that doesn't fit my hand and holds fewer rounds?

To me there was no debate. 43X. It's easily the most comfortable gun I've ever carried.

The 43 is a true single stack.

Glock will not make any bigger mag than a 6 round because they say  they cannot make them reliable.

I believe it.

The 43X/48 magazine is a little wider and very slightly staggers the rounds and helps to alleviate the inherent issues with single stacking tapered cases.

The light slide, stiff spring, and overall compact operating system are not as robust of function as the double stacks and are not as tolerant to fiddling.

Me?

I didn't find the 43X to really conceal much different than my 17K and I don't shoot them to near the same level.

When I want smaller it's for concealment so I use a 43.

43X/48 just doesn't have a place for me.

Robert.. I daily carry a G43 and originally tried a Taran +2 for the carry and reload mag (carried in a Tulster IWB holster and pouch), but found it printed more than I would like with a nylon cape back flats fishing shirt... I now use a Pearce +1`in the gun which prints less, but gives a much better grip on the pistol,  and still have the +2 for the reload...most of the year, however, I pocket carry with a Recluse two side cargo holster in a pair of nylon fishing shorts (in the outside cargo pocket... it takes some time to access the pistol, but the cargo pocket allows access when sitting that a normal pocket holster does not.... I'm just a kindly old retired school teacher with a low perception of threat....Les L747 Limited M

I have a 43, 43X and 48. If I could only keep one, it would be the 43. The 48 get the second most use. The 43X gets zero use.

I have the G43, G43X and two G48s (one DLC and one NPVD) guess which I carry the most? The silver slide 48 is at my side as I type this and the black slide is on my belt. I should have NEVER gotten the G43X. All I had to do is to replace the slide from my G43 on my G48 and viola, G43X......

I own a 43X, but would recommend the 43.  It might be related to my body type, but the Sig Sauer P365 with a 12 round magazine is easier to conceal than the 43X with a 10 round magazine.  I have always found a pistol's grip to be more of an issue during concealment than the slide.  If you like the grip size of the 43X, you might as well go with the 48.  My 43X has always functioned fine, but I have never really found a convincing reason to carry it over my other pistols.

If you appendix carry the Glock 43X might work but for carrying IWB at 3-4 O Clock I find the shorter grip matters.  Comparing my Glock 48 I find it prints less and the thinner grip helps in comfort compared to a Glock 19 IWB at 3-4 O Clock.   I find the 43 and 48 best combo for me.  If I really want a 43X I can swap my 43 slide to my 48.

I’ve got all three.... if I could only have one I’d prefer the G43X for my uses.

The 43 won't print as much if you're carrying strong side IWB, but it's not like the 43x is a difficult gun to conceal. I found the 43x to handle and shoot better by virtue of it's more substantial grip, and I also prefer front serrations. Great pistol.

I like being able to get three fingers on a gun without having to use a mag extension. The 43X suits me just fine.

This is just my opinion but whatever.

The 43x is an abomination.  The 48 makes sense.....with a 15rd mag.

The 43 is a different gun than the 43x/48.  It's a smaller option to the 19, not a replacement for the 43.  The 43 is the most concealable glock you can buy, whereas the 43x and 48 are more concealable than the 19 but too fat/long to compete with the 43.

Where Glock dropped the ball was putting a 10rd mag in the 43x/48 instead of going with a 15rd mag.  Sorry, not sorry.  I have a +2 taran extension on my 43 and it's damn concealable + if I want to really go stealthy I can slap the 6rd mag in it and it disappears.

And for that ease of use, I sacrifice 2rds to the 43x/48 in stock form.

Yes, the shield mag thing is a thing.  Yeah, I'm not sure I trust them.  Their roll-out has been sketch.  You have to replace the mag release as well.  Give it a few years and if it works well and Shield gets their shit together and is reliable, the mag release replacement will be worth doing.....but for now, well....that's that.