1 of these items you need to have, how to choose between them?
: you do it when your champion applies it easily like  or when your team has 2 or more AP champions.
: if your main targets don't have much magic resistance  and when you're a more utility mage, like , since you benefit a lot from the CDR status 
: when your main target has a lot of magic resistance  and you're an offensive champion that doesn't need as much CDR  or just needs more damage, like .

protobelt is a great first item currently, just don't do it if the match needs a , in case you're against ,  and  for example.
if you need to make , your first item will be  or  and the  of 2nd/3rd/4th item, depends on how the match is going.
 you do it when the enemy composition has a lot of health and the fights will be long.
remembering that, you can easily finish the build with both items.

great magic resistance item  in difficult matchups like  or in situations where it's very important to dodge a crowd control, like the first skill of  or ultimate of .