Depends on the situation. If you're tanking or hybridding, Amulet of Fury is the way to go. If you're in a situation where you just need mage (or want to hit particularly better with mage) then the Arcane Stream Necklace should be used. Depends.
Considering your saying what is better for magic that implys that you are using Magic Offensively in which case the massive Magic Attack bonus and the 15% magic damage makes it a very easy choice for Arcane stream.