Would you rather have 2% Life Steal or 5% Atk Spd as an ADC?

It depends which adc you're playing.

Lifesteal cause I don't want to die.

A bit of adc can go a long way. Rather take the doran's since the little HP can save me.

You'll need a common unit of measure to compare the two.

It depends on the champion. Typically, you go damage and attack speed when ahead and lifesteal, sustain, and defensive measures when behind.

5% attack speed gives more dueling power unless you have 150% or more bonus attack speed. 5% attack speed ALWAYS gives you more damage if you're in a situation where your team is protecting you and you are focused on dealing enough damage to carry the team.

Attack speed is pretty much strictly better.