Glass, definitely. But if you don't have a pipe suitable for glass screens, steel is better than brass.
Easy: None. I don't smoke out of metal pipes unless I'm sick of my bongs or my glass bowls are broken/lost (like right now). Currently I have a metal pipe for smoking on the go and I just throw in brass screens.. I get them for free at my headshop so what ever.. I prefer steel though, they dont give such a bad taste on screen hits when the bowls cashed. -AFD