I've got a double level ferret nation- it's great. Sturdy and roomy and easy to clean. The doors are big and can even come off so I've never had a problem getting anything in or out (some people have had issues getting wheels in through small doors). Plus many vendors on here make things specifically sized for either the ferret or critter nation.
Between the critter nation and ferret nation the biggest difference is bar spacing. Like Smidgeyy said the CN spacing is nice to protect the chins a bit more, and is also small enough baby chins cant squeeze through. The ferret nation does have sturdier bars than the critter nation though, so keep that in mind if you get heavy shelves and things- the weight needs to be dispersed more in a critter nation.
Ferret nations or critter nations can come up cheaper on craigslist occasionally, and you can also have 1, 2, or 3 "levels". On either style you would need to replace or cover the plastic shelving and remove the ramps. (also, Midwest is the manufacturer of these- I've seen some people get confused by that and think they were different kinds)
Martin's cages are also fairly popular (I don't think anywhere near the ferret nations though). I believe feisty ferret and all living things both make knock off versions of the ferret nation- cheaper priced at times but much cheaper quality.
If you order any cage online make sure you look over every part as soon as you get it. A few members have had issues with bent/broken/missing parts that make it difficult or impossible to put the cage together.
