Is there any advantage that glass has over HDPE (high-density polyethylene) for a secondary vessel? I have a 2.5gal batch in primary right now, and was thinking of transferring it to a 2.5gal water jug (same material as you get 1gal water jugs in).
Depends on how long your secondary will be. HDPE (which, despite what some people think, is not the same as the PET which Better Bottles are made of) has high enough oxygen permeability that it's not really well suited to long-term conditioning. At typical beer secondary times it's not really a huge concern, but if you're bulk aging a mead or wine glass is definitely the way to go.
Thanks. Per your comments, I think I'll make one half-hearted go at finding a 2.5-gal glass container, and will just use the HDPE if I don't find anything.
Well, yes, but that's not really germane to the discussion of HDPE vs. glass.