Hello. My first post, but I've been around quite a while. Contrary to my nomiker, I am a chemical officer nowadays, not Field Artillery anymore, and so I deal with masks quite often. Personally, I'd stay away from the M-40. Terribly uncomfortable for me anyway, and depending on when and by whom they were manufactured, some have faulty rubber. I would recommend the MCU-2. It's what I use, and have taken it through the live agent chamber at Leonard Wood many times. Unlike most civilian models, it is possible to drink while wearing it, which is important, and has a great field of view.
No matter which you get, make sure it accepts standard NATO filiters, keep extras on hand, remeber the C2a1 filter is the best on the planet, but above all else, remember that there may be no warning in the event you need it, and remember also that a mask by itself is not going to save you by itself. many agents absorb through the skin, so a chem suit may be something you want to think about also.
That's my $.02, anyway. Hope it's of some use.