I have shot the M18 and really liked it. Liked it so much that I bought a non-manual safety full size nitron finish P320. Liked that so much, that when I tripped across a used P320 X-Compact, I bought it. The P320 series, which the M18 is a part of, is a very modular and nice system. I have those two P320’s and the P365. For concealed carry, I prefer the X-Compact or the P365 depending on my clothes, where I’m going, and what I’m doing. The P320 X-Compact has their X series grip module which the main difference is a bit better grip due to the more defined beaver tail, optics ready slide, X-Ray3 night sights, and the X series flat trigger. I like the Sig flat trigger so much that I swapped the trigger on my P365 to the P365XL flat trigger, and my P320 full size to the X-series trigger to match my X-compact.
The X-compact and X-carry are specifically designed for concealed carry by Sig. They come with all the bells and whistles that Sig could pack into the gun for it to be ideal for concealed carry. I prefer the compact since the grip is enough to fill my hand, but not so long that it’s hard to conceal like the full size or carry grip length.