When to go Crit and when to go Armor Pen on GP

Hey there, Onwodori here.

Yesterday I had a game with GP Mid in which I went

and then hesitated before I built

The hesitation stemmed from my lack of other crit items - sure, when IE crits 20% of the time on a build like this, damn, they'll feel it. However, would have building something more defensive or been better by that point? I guess I'm looking for a good rule of thumb, when to split the build between crit and pen or when to double down on one of the other. I went Edge of Night early because I was against a Veigar and that's pretty self explanatory, but after that I could have gone crit.

Thoughts?

-Onwodori