Crit vs. Lethality ADCs

Lethality is better early on works on spells, so caster ADCs (which coincidentally are normally lane bully ADCs) usually work better with lethality.

Crit doesn't work on spells (there are exceptions), doesn't give AD beyond one item, and you need to build a lot of it to make it consistent. So it's better on heavy AS focused ADCs and amps up late game when you get more of the stat.

TLDR: Casters should generally go lethality, AS/right clickers should go crit. Exceptions may apply (Mainly Ez and Corki).