My 2c.....I almost live in these things.....I'm a medic and am frontline for a lot of stuff. The ones without the valves are fine. I have noticed that the ones with the valves do make it a little easier to breathe but that is not MY concern. I'm looking for complete 'respiratory isolation' for some pretty nasty stuff. For 'dust' and the like.....I wouldn't be too concerned about a perfect seal. I too agree that the "flat" masks don't have the strongest straps.....but I'll say this.....for dust.....I think I'd rather have a lot of the flat ones and change them often rather than 1 or 2 of the formed ones. I also 'store' the formed ones and can attest to the fact that they tend to start to (for lack of a better term) shred. They need to be verey protected or they start to pile (like sweaters do). The weave starts to loosen. I use both on a daily basis and would go with it this way (personally).....LOTS of the flat ones (aka duckbills) and good supply of the formed ones. In regards to the formed ones vs the ones with valves....you choose which.