The filter also makes the mask bulkier.
I just bought a box of each from Redflare. A quick test suggests that the ones with filters are undoubtedly better in use. They are both surprisingly easy to breath through. Without a filter, much of the air you exhale stays in the mask and forms a nice warm layer on your face. When you inhale, you get some of the same air you just exhaled. It's not as bad as it may sound but the valve is definitely worth having.
However, it makes the mask roughly twice as bulky. In many situations that doesn't matter. For example, I can put either into a jacker inner pocket and not notice them. In a draw or my car boot there's no problem. In other places it does matter. I have a small waist pack which is already stuffed with gear, and the valveless mask fits easily when the other doesn't. My rucksack has a thin map pocket which takes several valveless masks but I wouldn't want to put a valve one in. I also have a couple secreted in the door compartments of my car.
Currently I reckon they are both pretty wonderful, and much better than masks which don't fold flat. I am more concerned about dust than microbes, although I think they will help by stopping me from touching my face with my hands. They should help against fallout, too; you don't want radioactive dust inside you.
I think it's worth getting lots. You can lend then to other people, cache them all over the place, and have some to experiment and play with. Although it may look obvious how to put them on, there are a couple of gotchas so you need to read the instructions and then practice. (Eg I hadn't realised the lower bit was supposed to go beneath your chin.)
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Quality is addictive.