Armor pen CAN'T drop their armor below 0. However, early game is where Armor pen really shines. Around this time heroes have 30-50 armor, so negating 17 (all t3 armor pen marks) is quite substantial. That aside, a lucky string of crits early can also be the difference between a first blood and not. It really depends on the hero you play though. For Tryn, I would say Crit = Armor pen but say for Shaco I'd take Armor Pen over crit chance. Simply because his skills better compliment armor pen. Either way, I view runes as the early game decider. They don't have THAT much effect late game to be really considered but early game they make quite the difference. So, it's better to look at what you need early versus how things will pan out.

Armor pen 100%.

There is this fascination w/ crit early. You're not going to get a reliable crit chance w/o some serious gear. This gear does not exist early.

Armor Pen + Cleaver (last whisper is junk on trist), and you can bring pain to everyone all the time.

Crit dmg vs crit chance is a joke, and virtually no one has done the math right.

Every point of crit damage gives you a smaller % increase to total damage than the last, and there is an inflection point based upon your CURRENT crit chance that says 1% more crit = 1% harder hittin crits. I suspect its around 50% but im just guessing.

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.