EPub or PDF?

In My Humble Opinion
1

I’m preparing for some eBook downloads.
A lot of the O’Rielly books come in both both pdf and in a format I know very little about called EPub.
I’ve read up on both of them to some extent, but I haven’t really been able come up with a comparison baseline.

Can you fellow dopers help me?
Which one do you prefer and why?
Does one generally leave a smaller footprint than the other?
What are the strengths of one version over the other?
What are the weaknesses of one version over the other?

2

The big difference I see is that, with a PDF, you have a fixed page size, while an EPUB will be reformatted to fit whatever page size you need.

However, it seems that PDFs are mostly unchanging and here to stay, while EPUB is still discussing improvements and is young enough that it might be replaced. The Wikipedia article discusses a lot of criticisms I would expect to be improved, sooner or later.