In reply to DaveHK:
In short: Silnylon bags are all much of a muchness. They will develop pinprick holes and fail around the taped seams, usually with a regularly used lifetime of a year or two. This includes the alpkit ones, exped ultralight, seat to summit e.t.c
Thicker bags won't develop the pinholes, but will fail on the seams in a similar amount of time.
Thick canoe style bags - like the Exped Heavy or Ortlieb are durable but just not really worth the weight.
If you trust it you can go for a solution that just has a heavyweight liner for your sack, then not care about the absolute waterproofness of the internal bags and go lightweight.
Personally, I'd just buy the lightest thing on offer and accept that I'm gonna have to retire them to not being absolutely watertight after a year or two (or just accept it as only a tiny amount of dampness would penetrate anyway). It's only absolutely crucial if your life depends on it being dry e.t.c