Precision Mastery: 3 + 0.3 per level Magic Penetration and Armour Penetration Intelligence Mastery: CDR cap increased to 45% and you gain 5% CDR
I'm just wondering in what situations would you choose one of these masteries over the other; for example, when playing mages or mage-like supports.
Well, it depends on how you play the game, and what you want to do.
For heavily CD-gated champs, like Fizz and other assassins, it helps them pull off their combos more frequently, and in Fizz's case, it lets him use his troll pole even more. So, if you want to go for that kind of playstyle, that's where the 5% extra CDR would come in handy.
On the flipside, having extra pen helps if you don't like building pen items, and it helps with early game cheese kills as well. Also helps to whittle down tanks a bit, since MR pen is a bit lacking.
So, both are good in their own way, and both have their own use. How you choose is up to you, and depends on what you want to do with the game.
If you're going support always go for the CDR, you're not going to stack damage anyway, and you need to use your base damages, also since you have easy access to mana regen you will do more damage if you cast more spells rather than casting a few big ones, on supports it gets ridiculously useful (example, thresh at 45% cdr can cast a hook every 5 seconds, think about that, also thunderlords gives you a tremendous amount of kill pressure in lane/ damage deterrent ), same goes for more damage oriented supports such as karma or annie, even zyra or brand that have a more "over time" kind of damage.
If you're going a carry, even orianna, karma or lulu that are basically farming supports, take the pen one instead, unless you want to give up a portion of your damage to favor very low cooldowns, even if it doesn't come to my mind any, i would rather always go pen if i'm playing anything that isn't support.
