Step 1: Do you only want High Isle? Or do you want previous chapters as well?

If you only want High Isle buy the Upgrade.
If you want all the previous chapters as well buy the Collection.

Step 2: Do you want the collectors edition bonus items? (The emotes, ornaug mount, senche pet, ascendant knight armour and treasure chest memento.)

Yes - Buy one which says Collector's Edition
No - Buy one which does not say collector's Edition

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Collection: High Isle Collectors Edition - this is the full package - High Isle plus the base game, Morrowind, Summerset, Elseweyr, Greymoor, Blackwood and the collectors edition bonuses for High Isle.

Collection: High Isle - the complete set of chapters without collectors edition bonuses. You get High Isle plus the base game, Morrowind, Summerset, Elseweyr, Greymoor and Blackwood.

High Isle Collector's Edition Upgrade - Only for people who already own ESO and have or don't want previous chapters. High Isle and the Collectors Edition bonuses but no previous chapters.

High Isle Upgrade - Only for people who already own ESO and have or don't want the previous chapters and don't want the collectors edition items. This is just the new chapter and nothing else.

All versions come with the pre-order bonuses if you buy before the release date.