I think I am going with the 17 mil billboard tarp liner. I feel like it should be fine for the short-term -- as in the 3-4 years (at most) I have left in my current house. After that, it's the next owner's problem. The difference in price -- $60 shipped compared to $130 shipped for a 22 mil liner) is drastic, given that the difference in thickness is relatively small.
Thanks, all, for the help. I'll post some pictures when the project is done. Probably won't be until I return home from Disney in the late part of May.