Route. Schedule. Price.

Remember, not everyone is a FTer and I doubt you'll see a mass exodus of customers from one airline to the other.

What you will see is customers booking partner non-stops over connections and I'd wager the average customer (not FTer) would rather endure a non-stop in 31" Y then a connection for E+ or F.

In other words you might see a SFO based 1K opt for a CO flight to CLE or a WAS based Plat choose a UA service out of IAD rather then connect or trek to EWR.

But in any event I seriously doubt the majority of customers of either airline will go out of their way to take a connection over a non-stop for E+ or upgrades.