Should I go crit or lethality mf

I'm beat as MF for the ADC role. I don't play alot f ADC making her the perfect choice for me. I consistently carry games. I've been building crit after the buffed crit mf but people on mobafire still recommend lethality. I will still build edge of night either way so they can't cancel ult. I go ie, rapid fire cannon, death's dance/bloodthirster ,edge of night, phantom dancer, and of course boots but if eon isn't needed, ga. For lethality,I don't get any crit items. I get youmus, duskblade, boots, eon, ga, and death's dance. Which is better