ER is a IE supplement, not replacement. If you get ER, you always want IE.

If your champ wants AD, crit, and CDR, you get ER. You then still want more AD and crit, and IE is the only other option, asides from trinity, which gives 40 less AD.

Some champions will get TF after ER, which gets them to 40% cdr and crit. At that point, you want IE to make those crits hit harder, as well as any spells with AD ratios.

Others will go straight from ER to IE.