Throughout my experience with all things rusty, dusty, dirty and nasty, I have found that a half face respirator from 3M (with rubber cup that seals around your mouth and nose, cartridges and adjustable head and neck strap that has a easy release clasp) are far superior to any disposable paper/gauze like thingy.
This may be due in part to the fact that I keep facial hair around my chin; it holds the disposables away and does not make a very good filter. However, I find that the rubber cup of the proper respirator seals much more positively (as confirmed when test fit by my work); the straps can be adjusted (unlike the disposables that always seem to be too tight, even though they don't seal well); they are much easier to put on and take off without tangling in your hair; and they don't fog up your glasses because the exhaust vent is directed downward (whereas the disposables leak and fog).
Do yourself a big favor and spring the extra money for the respirator. They last a long time and they work.