I think 'Absolute' is a marketing term invented by DC for marketing reprints. When they reprint the original plus every peripheral piece of work that they can find and justify, they call that an 'Absolute' edition. I think the first was 'Absolute Watchmen'. Marvel uses 'Omnibus' now for single collections of 'runs' of titles when the collection might run into 20 or more issues. Reprints are currently big business for the 'Big two'
So if you just want to read, Omnibus. If your interest is greater ( or if you are a hard-core fan) and you want to collect everything about that work, Absolute is for you.
Cheers.