Both versions would be correct grammatically and idiomatically, though I personally prefer which.
Which is generally used when selecting from a number of items in a group. Here we are choosing from a variety of potentially suitable materials. If you are presented with a limited set of choices and asked to choose one, which is strongly preferred.
What is more open; it allows you to select from a much broader range of options. If you are presented with a broad choice, what is preferred. What is the only option when no limits have been placed on the choice.
Ex: My wife and I are trying to decide what we want to do with our weekend.
Me: What would you like to do today?
In this sentence what is the only option. Without some (even poorly defined) set of possible choices, which does not make sense.
Ex: I am at a restaurant with my wife and we have just started looking over the menu.
Me: What do you think you'll order? (broad range of choices)
Wife: I'm not sure, the fish and the chicken both look good to me. Which would you pick? (narrow range of choices)
In my statement, what is the proper word, as all options are available. Using which here would be confusing (though perhaps I could accompany it with a gesture toward the choices on the menu and force it to make sense.) In my wife's statement, either which or what is appropriate, though which would be preferable. Moreover, if my wife used what here, she might be asking my opinion from the entire menu, whereas which limits my response to either chicken or fish.