I have both as well. I carried the CM9 for several years and as it's been such a reliable pistol, I plan on keeping it. It won't see any carry time unless the P365 has to go in for repairs.
The overall size is quite comparable (see photos below for a brief comparison).
The 4-round advantage out of a pistol the same size as my CM9 was what drew me to the P365 and compelled me to order one. After receiving it, I found the capacity advantage wasn't the only thing the P365 has going for it.
For one, it has a better trigger... in my opinion, of course. It's about the same weight, but the travel is not anywhere near as long. Whereas the CM9 trigger feels like a decent double action revolver, the P365 has a much shorter pull (and a
far shorter reset) approaching the feel you would expect from a single action.
Considering my method I carry (Kydex IWB holster), this is an advantage.
I wouldn't recommend this (or anything else) be carried loose in the pocket as a primary method. I require the trigger guard to be covered/inaccessible during carry, but that's just me. A pocket holster such the DeSantis Nemesis costs less than $20. It also breaks the profile of the pistol to reduce printing and keeps the pistol in a position ready to draw. There's no reason I can think of not to use one. I'm guilty of tossing an NAA Pug or P32 in the pocket to fetch the mail, but this isn't the safest nor efficient way to carry if you're heading out for any length of time.
The ergonomics are far better, for me, with the P365. This is one of those things where if you never felt one, you don't know what you're missing. I was perfectly content with how the CM9 felt in the hand. However, after handling the P365, I discovered the CM9 just didn't fit like the P365 does.
The sights are superior on the P365 as I find, again for me, sight acquisition is better with the P365. The difference is only amplified in lower light conditions were the P365's standard front and rear tritium night sights come into play.
They both point the same. This being when I point at the intended target, the front sight is aligned right were I want it. So point-shooting is about equal in this regard. I don't see one having an advantage over the other when it come to the amount of time it takes to draw and put a round on a nearby target.
Likewise, I don't really shoot one significantly better than the other. This said, I have owned the CM9 far longer and put many more rounds through it than the P365. Over time, this may change.
One thing I haven't seen mentioned when comparing the two is that the P365, though rated +P by Sig Sauer, has a slide that is significantly easier to rack then the CM9. I don't know the mechanics behind this but it is noticeable to me. I've never encountered an automatic where the slide poses a challenge to pull, but many new or female shooters may find the P365's slide easier/more comfortable to manipulate.
I haven't had a bobble in either, so both to date have been perfectly reliable.
Better ergonomics, better sights and far superior round count... it's hard to contest that the P365 is a better CCW option for me. However, just as 460 Shooter, Franco45 and Jeff H stated above, I'm keeping my also excellent CM9 as a "back up" in case something should happen to my P365.
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