One consideration would be how you plan on carrying it. I've previously owned both 640's and 340's in the past and found the 640 wasn't ideal for pocket carry while the 642/442 pretty much hit the sweet spot for me in terms of weight, cost and proven dependability. I have two 642's and do most of my hard training with an old beater 442. Most people do find the 640 more pleasant to train with and less so with the air-weights, but how much depends on the individual and I personally don't find it prohibitively unpleasant to put as many rounds down range as I want to with the 442. Plus I like training with a weapon that is identical(aside from color) to my carry gun.

I honestly don't spend too much time worrying about active-shooter scenarios because despite all of the fanatical media coverage they receive, they are an exceedingly rare occurrence and there are much more important and common potential personal defense encounters to focus on. However, if you are one of the select unlucky few to be in the wrong place at the wrong time and there is a substantial gap between you and the shooter, my recommendation would be to get yourself and your family away from the scene as quickly as possible. I for sure wouldn't recommend getting into a long range shootout against a homicidal/suicidal individual armed with an AR or AK when all you have is a snub revolver if you can avoid it. My recommendation would be the same even if armed with a Glock 17. In very close-quarters, the dynamics probably wouldn't differ all that much from any other defense scenario and there isn't going to be much difference between the 340 and 640 in such cases except maybe the latter would offer a bit more recoil management and perhaps make for a slightly better striking implement if things were to become that desperate. I travel to many of the largest cities in the country on a regular basis and feel well armed and well served by my 642.