I own one of each (one FN and one CN). i got the first FN because it's what PEtsmart carried in stores (thus saving on shipping costs). the second time around, petsmart carried only the CN so again, thats what i got.
i didnt mind getting the CN for my second cage because I realized that while I wanted the cage for chinnies NOW, i might want to use it for something smaller in the future (degus, sugar gliders, etc) and the bar spacing is smaller. yes, this smaller bar spacing prevents me from petting the chins thru the bars of the cage, but it just makes me actually open it up and give them better scratches!
the bars on the CN are horizontal (and the FN are vertical). it DOES make it harder to screw ledges on, but i was able to place all my ledges at points that had both vertical and horizontal bars, which made them more stable.
CONCERNING WHEELS... if you plan on getting a good chin-approved wheel for your chin.... please note that the "silver surfer" wheel WILL NOT work in a CN, but a "Flying saucer" will (due to bar spacing). either wheel will work in a FN. i have both cages and both types of wheels, so i know! (i do not have a chin spin, so i dont know how this one would work in a CN).
I own 3 ferret nations & 1 critter. I prefer the FN over the CN any day. Imo the only benefit to the critter is the bar spacing for young chins/kits which you can accomplish w/ hardware cloth on the FN
The 3 FNs I have are attached with hardware and my critter pops together, I would Never recommend the pop together- mine leans to one side & looks like its going to fall apart at any moment! I also have had 2 bars break on my critter and Im positive more will break, the bars can barely hold up straight my smallest ledge. To be completely honest I would call my critter nation a piece of crap and it costed more than the other cages!!
it was my understanding that the "pop together" cages (whether FN or CN) were the newer version of the cage, and the hardware cages were the old version... is this not correct? my FN was bought about 5 years ago and was built with hardware (very sturdy) and my CN was bought less than a year ago, and is "pop together" (not quite as sturdy, and harder to build, no doubt). if i am incorrect, and you have the choice, i would DEFINITELY choose the "build with hardware" cage.