Eternal Items: (Green Items) these items have an Item Quality of 50%, which increases the Damage of Weapons or AR of the other 4 items. values of affixes are doubled. Strength of Ritual Beads is +10 max, but on Eternal items is +20 max. if it has a socket, you can put in a Myth Stone, but the MS values are not doubled. if there is an affix that is on the list for Obsidian Crystal to be a Crystal affix, there is a chance for 1 of those affixes to be Crystal on an Eternal item if it is normally on that item. Crystal affixes are not doubled on Eternal items, as they are already tripled. you can’t increase the level of Eternal items, except for Eternal Pets. here is a post on how hard it is to craft an Eternal item.
Legend Items: (Red Items) Legend items are premade items that you can find. some are solo items, and some are part of a set with a theme. their Item Quality can be improved to 25%. they all have at least one Legend affix (Red). the thing about Legend items is that they are great for crafting. as long as the item is a Legend item, when trying to reroll affix values, they wont usually go below 75% of the max value if the items level is high enough.
Crystal Legend Items: (Cyan Items. Eternal items with Crystal affix are still green) these are Legend or Eternal items with a Cyan, or Crystal Affix, on it. there can only be 1 Crystal affix on each item. there are about 20 Crystal affixes. all of them except for HP/MP on Hit (double value) can get up to triple value. Attack Speed +15% (Epic affix - Orange) would be Attack Speed +45% (Crystal affix - Cyan). Legend or Eternal Items can be found randomly with Crystal affixes, or you could use Obsidian Crystal to put a Crystal affix on an item that isn’t a Pet or Eternal.
basically, if all or most of the affixes can be used in your build, then Eternal is best. Crystal items are easier to change affixes on if you only need a few affixes on the items and want to put others instead.
hope this helps.