To me, a true mousegun is one that literally disappears into a front or rear pocket, even if wearing tight jeans, one that you don't even know is there until you need it. Size and DEEP concealment, in a caliber that is regarded as being at least minimally effective (which for me means .380ACP) are my primary concerns in this category. In my book, the guns you mentioned originally qualify, as does the Keltec P3AT, the Taurus TCP and the Kahr P380.
To me, with the exception of the
Rohrbaugh R9, I have not found anything in 9mm Luger I would classify as a true "mousegun". Yes, the PM9 can work as a pocket carry 9mm. But compared to the others it is no mousegun. I find I always know the PM9 is there, both in size and weight, when pocket carried. Neither is the Keltec PF9 really a mousegun in my book, although it comes close. To me, however it is still too big and heavy for true mousegun-like concealment.
I've owned a first generation P3AT and I now own the Ruger LCP. I liked the P3AT but like the LCP even better. I find the LCP to be more polished and refined, and I have found the reliability to be better than the first gen P3AT that I had. I carry the LCP on-duty as a second backup weapon (#3 handgun). I can't justify the astronomical price of the Rohrbaugh R9, and the Diamondback is not an option for me as it is not an approved manufacturer by our department (neither is Keltec for that matter).
I'll agree, the LCP is not the most pleasant gun to shoot, it does take some getting used to, and the sights are nothing to write about. But it has been reliable and a good shooter for me. I have to dispel the notion that the little LCP is inherently inaccurate just because it is a tiny pistol. Each year I have to qualify on a 50 round course with it, and each year I do so all the way out to 15 yards with ease. Last year just for grins, I tried my hand at 25 yard paper black silhouette targets with the Ruger LCP and Sellier & Belliot ammo. Guess what? All seven rounds in the torso at 25 yards, firing timed and two handed from behind cover. I'll take that level of accuracy from my "Noisy Cricket" at that distance! While I would never recommend the LCP for these longer ranges as a matter of routine, I mention it here simply to indicate that it can be much more accurate than it is sometimes made out to be. For the fit and finish of the LCP, the price paid and the fact that you have the support of Sturm Ruger behind it if you need it, I don't think you can go wrong with the little LCP. I opted for the satin stainless version which I find deals with hot and sweaty pockets during summer carry better than the blued.
That being said, a final comment on revolvers. While I really like my little LCP, IMHO none of these mouseguns are in the same category with regard to reliability and performance as a J-frame revolver. My J-frame S&W 642 is not a mousegun by any means, holds only 5 rounds and will never compete with the LCP in terms of weight and size. And that is OK, I bought them each for carry under different circumstances and each fulfill their role nicely. But I honestly believe I will witness icicles hanging in Hades before I see a jam or misfire with that 642, and the performance of the Speer +P Gold Dots or the Remington R38S12 "FBI Load" is superior to what I have seen coming from any .380 ACP round thus far.