I love my FN, but I also know others who love their QC mansion. The decision is ultimately yours. Do you like your mansion? Also, are you willing to take a cut in the price you paid for it? Used isn't going to get you what you paid for new.
If you have a QC Mansion, you should be aware of the good points- and the bad. Which to buy is a good question, but switching just doesn't make sense to me, $ or quality.
I have three QC's and my last cage was the FN double. I really like having the two doors on the FN open. But I won't buy another. The metal pans are hard to move in and out. I like the QC cages as the pans are very sturdy. I buy the large one's for the huge double doors. That way I can reach in to the top or bottom. I like the fact that the entire top comes open for the large wheels. I love those powder coated pans. They clean up fast and they move in and out like a charm. And I like the fact that the cage folds flat to move. The FN is really very heavy. However I wish the CQ's came with the darned stands. Both are nice cages. And now that I own both, I will go back to the QC.