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As spoiler free as I can make it, Act 1 is it's own campaign with begining middle and end that will take you from LL0-5. Act 2 instead of taking the ending of act 1 goes down another path to continue the story.
You don't need to own both to get a complete product or campaign, they just connect to one another.
Assume you mean the books with full art and all the pages, the one with the larger file size is uncompressed and has a lot more fixes. Basically if you want the most up to date version, use the one you've downloaded of here