For ponchos, silnylon will keep you dry from the outside. It's not breathable, though, so if you're sweating, you will stay wet from the moisture you're producing.

If you aren't bush whacking, driducks ponchos are [censored] near waterproof, and breathe well. I like them quite a lot when I'm hiking on a well worn trail, but they suck if you go off trail. The problem is, they are pretty fragile, so if they get snagged on a branch or something, they will probably tear. If I'm carrying a driducks, you can rest assured I'm carrying a bit of duct tape too.