Originally Posted by Pink Princess Party
Having a higher crit chance is going to affect your game overall more than Penetration. If you need penetration, Last Whispers and Brutalizers are cheap, and the speed you gain from a Last Whisper is going to help against both casters and tanks so you might as well grab one.
Being able to crit early game is WAY more important than Armor Penetration. Just having 12% crit with one BF sword at Level 6-8 is worth more than all the crit chance in the game IMO.
Also, if you choose a lower damage build like Frozen Mallet/Last Whisper Crit is going to up your damage just that much more, and your gonna wreak havoc on katarina's trying to run away after you pop em out of their ult with a buster and they're already slowed for you to just pin down.
Also you can get to the point early where if you go with a zeal build you can have 1/4 of all attacks critting super super early and thats huge for the farming advantage which is what tristana can really get her punch from.
Mathematically, Armor Penetration is going to do more DPS increase than Crit Chance. For the entire game. It is also more consistent.
Yes, having 20% crit means that your attacks, on average, deal 20% more, but each individual attack has an 80% chance of just doing normal damage. While the attacks made with 20-something armor pen are going to be doing significantly more damage on every single attack, and enough more than it makes up for the fact that you have fewer attacks getting "lucky" for twice as much.