I don't have any of these trailers but have looked at them, different models though.
Other than the azdel walls, the major difference I noted are in the axle/wheels. The Keystone Passport & Cougars have heavier duty 15" tires and heavier rated axles giving you more of a "cushion" there.
Also the Keystone Passport & Cougars have heating ducted through the underbelly with floor vents. And I suspect they have better 4 season capabilities.
By the way, if looking for used, I think the freedom express just started using Azdel in 2013, not sure about apex.
EDIT: and the Apex has the Select and Standard series which are quite different trailers