Eternal Items (Green) double the values of affixes and have an Item Quality of +50%. as you can see in the post I shared, only a few Crystals can be used to Craft/Edit an Eternal Item. you are basically using an Eternal Item as is, with the ability to make some minor changes. the +50% Item Quality and doubled affix values is what make Eternal Items so powerful, but their weakness is the inability to make major changes to the item.
Legend Items (Red) are found with an Item Quality of -10% to +10% and can have the IQ increased to +25% using Emeralds. with a lower IQ and affix values not doubled, Legend Items are weaker than Eternal. the strength of using Legend Items is that you can use any Crystal on them to Craft/Edit them however you want, which can make them stronger than an Eternal Item.
using an Eternal Item or two in a Build is good if most or all of the affixes are useful in your Build, but the flexibility of using Legend Items makes up for Legend Items not being as powerful as Eternal Items.