Question about particular players build (new to panth)

So I was browsing league of graphs and I came across this player NalgaDePastelero#LAN - Pantheon performance - League of Legends. I like how he builds and I understand most of his item choice except one. How do you know when to go sundered sky vs eclipse (pantheon support) it seems most of the time he chooses one or the other but they're very similar in my head.