~338 AD with 25% crit chance against:
~318 AD with 50% crit chance.
It hits harder, it's actually safer although that is for assured damage and not losing BT stacks, whilst lifesteal gives far more utility than two IE's can provide, HP stackers beat it hard, Armor reduces your damage significantly and in building a second IE Lifesteal will probably be needed along with the Last Whisper hence Earlyish Vamp sceptre to allow for late game sustain with your IE and with items like Zephyr, Runaan's Hurricane and Sword of the divine, a lot of carries work better with flat damage, those who don't often dominate with Sword of the Divine or Phantom Dancer and on top of them all is the Zephyr, giving almost any carry insane potential for actual damage dealt, rather than theoretical raw unimpeded crits at 50% chance.