I don't even know if my line of thinking works... or could I probably be comparing apples with oranges.
How about this... would it be better to sacrifice for upgrade an item which starts out as an epic (ex. desolation) or an item which starts out as a rare (ex. corrupt light). Let's say for discussion's sake both are drops from free boxes and are at Level 1 epic.
Which would you sacrifice first? Assuming you only have one of each, so you can make a legendary.
In terms of rarity, I probably would not sacrifice the epic-starting item.
However, in terms of power... I don't know. Would they be of the same value since both are of epic level? Or... as I have mentioned above... Am I even doing the right comparison?