A good source of information about what is in any product on the market in the United States is the "Material Safety Data Sheet".
If you put "Glad Plastic wrap MSDS" into a search engine, you will quickly find:http://www.thecloroxcompany.com/downloads/msds/glad/gladwrap.pdf
which will tell you that the material is Polyethelene.
I didn't find a Saran Warp MSDS; however, the SC Johnson Company prides itself in disclosing what is in all of their products, it just takes a little more searching. On their Saran web pages, there is a FAQ What ingredients are in Saran ™ Plastic Wraps?" that states it is polyethelene.
From a food safety standpoint, polyethelene is likely to be the ONLY material used by all the plastic wrap manufacturers.
If you search for "permeability of polyethelene", you might find an old Bell Laboratories article from 1966 http://www.alcatel-lucent.com/bstj/vol46-1967/articles/bstj46-2-391.pdf
that offers some hope of finding differences in permeability of water vapor in various polyethelenes; however, the competitive nature of the pastic food wrap business might have driven the major competitors to provide similarly performing products.
There are some high end commercial restaurant supply vendors that sell thicker polyethelene food wraps at a higher price point that may have a lower permeability.
In any case, the permeabilities of polyethelene are low enough that any seal leakage around the edges is likely dominate any test results and your test proceedures are going to have to figure out how to eliminate that leakage if you really want to test permeability of the material.
This line of thought leads to an alternative experiment that ignores the permeability and tests for how well various manufacturer's "cling wrap" seals to identical containers. If you head that direction, you will have to control the cleanliness of the surface of the identical containers. Any skin oil or other surface contaminant could alter the effectiveness of any plastic wrap product.