If space is a primary consideration for your BOB, then you can do without a valved version. The valve is primarily for comfort and a model with or without a valve will have an equivalent filtration capability. However, depending on how you envision using them, perhaps comfort could also be a major consideration. I live in earthquake country and even after the ground stops shaking, I may want/need to wear a mask for an extended period of time, day after day, to work amid concrete dust and other kinds of dust. If exteneded wear is a reasonable scenario, then a valved model would be a nice luxury. Actually, I personally just keep regular dust masks for the earthquake scenario, since an N-95 mask seems overkill for that kind of use.