Hmmm.
I voted 3913, because when it comes to a lawfully concealed belt gun it's been my primary favorite among compact S&W pistols.
FWIW, I actually own a 3913, 4013TSW, 4513TSW, 4040PD, SW99 compact, CS45 & CS9. I've trained & qualified with all of them and have carried all of them at one time or another, so I 'voted' with some small experience in mind. (My M&P is a full-size, but I've considered picking up a compact at some point.)
Just to show that I'm somewhat conflicted, however, I've come to enjoy shooting the SW99 compact (9mm) more than the 3913 and over the last couple of years. I've come to consistently experience better practical accuracy with the little 99 compact. Not sure why, really. That said, I'd still choose the 3913 over it for a belt gun.
Now, if I do pick up a 9mm M&P compact sometime and it's anything like my M&P 45, I wouldn't be at all surprised to eventually find myself choosing the M&P 9 compact over all of them. Dunno. Not there yet.
One model I'd suggest as an addition to your poll list would be the discontinued 6906, though. I've fired many, many thousands of rounds through one or another of them over the years and have always been very satisfied with them. Just not as slim of a choice as the thinner single column 3913.