That's one of the more important distinctions as Wasabi Tofu pointed out....when you're Emerald in OW you get access to First lounges in any place they exist no matter the ticket or fare class. It really does make a difference because anyone is Gold in *A, not difficult to attain at all, but Emerald is really beyond the reach of a casual traveler and when your flight gets re-routed the partner airlines really take care of Emeralds. Also, I've found Cathay to give excellent treatment to Emerald's - no matter the class of service. As well, I feel there are a lot more fare rules on *A when you take partner airlines in regards to mileage accrual. OW I find is a lot easier, you basically garner EQM if you're an elite save for some specific instances.