Simply two different cruise experiences. I like both classes of ship. I think Freedom offers a bit more of a relaxing cruise. On O class ships there is aways something going on and I don't get much down time.
I have Freedom booked next because it is an 8 nighter and 4 ports with no private stops. Those cruises are harder to come by. I've enjoyed every cruise that I've done on an F class ship. However, if I had the choice of Freedom or Odyssey for the same cruise I'd most likely choose Odyssey.