IF, big IF, I was going to buy one I'd pick the M&P folding PCC over the Keltec.
Check it out. Uses M&P magazines if you've already got them and carry a FS M&P 9MM pistols.
Barrel folds sideways so a red dot optic (not necessarily pistol sized either) and back up sights would be easy/workable. Where do you put the optic on a Keltec?
My 9MM PCC are AR style (that consistency thing that lives in my head).
The Ruger PC9, all being takedown models are nearly as compact and a more robust design than either, including all metal at critical junctures. Go w/ the charger version if you need uber-compact. Mostly same controls as 10/22.
I have a couple sub2ks and do do some reliability and improvement mods to them, but even modest mods you're turning a $400-500 gun into a $700-$800 one relatively easily -- the Gen2 Sub2k definitely shaves off some cost due to improved front sight and stock adjustment, but not much in the way of critical parts. I generaly upgrade feed ramp, rear sight , trigger springs and trigger bar, charging handle, metal trigger, tube cover, buffer, heavier bolt upgrade depending on how the gun recoils and what I'm shooting in it, screws and posts, etc... If I didn't already have a bunch of spare Sub2k parts, I'd go either PC9 or AR9... B/c metal-central, I would also trust both as trunk guns as well -- not ideal for the role, but could definitely take the Texas heat.
At the time I got into Sub2ks when still their Gen1, Colt was the only AR9 lower on the market (best as I can recall) so by the time you added the Hahn magwell insert to all the other options and dealt with all the other unique 9mm features and headaches, it was still cheaper by a couple hundred dollars to mod the Sub2k (and 9mm ARs w/ barrel options in rifle length were significantly heavier than Sub2k as well), and I also didn't own any other ARs at the time of the first Sub2k acquisition... Now, 9mm ARs are ubiquitous and ARs of all sorts are much cheaper and easier to build as well, and I have more and have spent more time n ARs. Glock 9mm ARs also weren't really a thing best as I can remember.
Depending on the pistol brace rule outcome as well as whether there's a lightweight factory or aftermarket barrel option for the Ruger LC carbine, I might offload the Sub2ks, even if at a loss. Regarding pistol brace rule and bought my first Sub2k before those were on the market -- the compact nature and balance due to pistol grip mag, was huge appeal and desired feature of the Sub2k vs other options at the time... However, I increasingly view, given other options (Ruger's PCCs and PCC ARs) that we have now, that upgrading Sub2ks as putting lipstick on a pig these days.
And except for ammo cost and my caliber inventory, as well as weight due to its unnecessarily heavy barrel -- the LC carbine does everything I purchased the Sub2k for (that primarily being for a compact HD gun) better. And I only see the S&W carbine addressing a couple of the issues w/ the Sub2k's design -- certainly not all of them -- although is probably a better consumer gun since it doesn't necessitate as much DIY "gunsmithing" out-of-the box.
