Good question. Not much difference, to be honest. Most bands that are labeled emocore, are labeled posthardcore as well. Also, McKay said he was actually kidding about using "emocore" as a way to describe music.
However, the bands that use "emotional talk", sobbing, etc (Intohimo, La Dispute) tend to be referred to as emocore the most.
Bands that use clean vocals combined with screaming are usually referred to as posthardcore.