All white flours, both unbleached and bleached, go through some bleaching action except that in the case of unbleached flours the bleaching is done naturally, through the exposure of the flour to air (oxygen), typically for an extended period of time, whereas chemical agents used to bleach flour act considerably faster. Unbleached flours are yellower and creamier in color than bleached flours because there is less damage to the carotenoid pigments when exposed to air. Chemical bleaching agents destroy more of the carotenoid pigments, making the flour whiter than unbleached flours. It is also believed that oxidizing flour destroys the Vitamin E that is in the fats in the flour and thus alters the flavor of the end product, such as bread, made using the flour. Destruction of the caretenoids in the flour can also have an adverse effect not only on the color of the finished crust but also on its aroma and taste. Some flours, like cake flours, are almost always bleached because the bleaching strengthens an otherwise weak flour and gives the dough better volume, although a couple or years or so ago King Arthur introduced an unbleached version of its cake flour. According to Tom Lehmann, much of the flour used by the baking industry today is unbleached. Given a choice, for pizza dough I would go with an unbleached flour if it is an option since whiteness of the flour is not something that is needed to achieve a high quality pizza crust.
It should also be noted that at the professional level, bleached flours quite often tend to also be bromated. However, there are bromated flours that can be unbleached although they may be harder to find or have to be special ordered.
In your case, the choice will ultimately depend on what kind of pizza you want to make and what flours are needed and available to make that kind of pizza. But if you are at your supermarket or somewhere else and you have a choice between bleached and unbleached versions of a given flour, you might go with the unbleached version.