I have a Feisty Ferret (FF) cage. I wanted the FN, but my Chin room was too small. The FF fit perfectly in the corner of the room and I only have 1 chin, so that pretty much made the decision for me.
With the FF, the middle wire shelf can be left in or removed, making for a nice sized space, especially for 1 chin.
The FN is obviously a better cage for the reasons already posted. However, if you have space issues, the Feisty Ferret is not a bad cage. I don't have a problem with the single doors, but then again I only have 1 chin, and am using a fleece liner so its easy to switch them out.
One thing to caution with the FF cage though, the weld spots are not too thick. I read that some people had issues with breakage using heavy wheels like the Chin Spin. While mine has been fine, I can definitely see how that can happen. I re-enforced my Chin Spin with aluminum bars to take the weight of the cage bars.
If you have the space, I would get the FN.