We've stayed at the World Center and Swan several times. Overall, we did not like the OWC. It is offsite, the decor is boring (country), but the pools were nicer than the Swan. As a Marriott Gold Level, we did not received any benefits.
We really enjoyed our stays at the Swan. It's a Westin hotel, so they have the Heavenly bed and bath amenities. I'm also a Starwood Platinum level Preferred guest, so we were always upgraded to a jr. suite or larger. Also, they provided club access for free. The hotel is also very close to Epcot and MGM.