Oh yeah, sil will pass thru moisture; extremely irritating when it's cold and wet out and you've got nice warm, dry clothes on as you climb into your hammock.
Go with the 'more expensive' options if you are even kind of concerned with long life and weight. Just be aware that you will, at some point, be mad at any tarp you buy; nature of the beast. If you feel like spending a little more coin, Brian (OES-MacCat) will make you a tarp of Spinntex. Oh man is this stuff sweet! You won't have to worry about moisture pass-thru, crazy lightweight, and so far, very durable.