You didn't share the route, but I would look also at the ease of connection.

If you're changing carriers in JFK, it's likely that you will need to change terminals, which means exiting security and reclearing. This is not the case at IAD where everything is behind the same security. ORD has a lot behind the same security, but some carriers in T5.

On the inbound to USA it'd be less of a concern as you'd have to reclear security regardless.