which ferret cage is best?

I don't know much about the Marshall cage, but I've been using a Ferret Nation for several years now and have had no problems. It's plenty big enough for more ferrets if you ever decide to get more, and even if you just stick with one I'm sure he'll love all the extra room! It is true that you'll probably need more litter boxes, depending on how well trained your ferret is. I have four in my cage, on on each of the "mini" levels, and two on the two right corners on the bottom. My ferret likes to poop in any corner, regardless of whether or not there's a litter box, so I just made sure to put food/water dishes, bedding, or something else down to block the corners.

The quality of the Ferret Nation cage is excellent. The only problem I've run into is that the doors will come out of their hinges if you bump them too hard. And by hard I mean accidentally standing up underneath them - something I very strongly do not recommend. Ouch! :p But it's really not a problem because they go right back on and are generally very sturdy. The ramps are nice, too, because even if you don't put covers on them (I believe the Ferret Nation should come with some when you purchase it), it's extremely difficult for the ferret to hurt himself on them - the bars are too widely spaced for him to catch a toe/nail on, but just wide enough for him to be able to scamper up with ease.

My favorite thing about the Ferret Nation is that you can seal off the levels from each other. There's a little hook that you can attach the connecting ramp too, to block off the second level from the first. Very handy if you have to clean the cage, but can't take your ferret out of it.

Cleaning the cage is an entirely different story. It's massive, and the material the trays are made out of isn't very conducive to easy surface cleaning. Be prepared to spend a while on a good cleaning.

I would pick it from your ferret's potty habits.

A Ferret Nation does not work well with one with poor habits. You end up putting more potty boxes in it and they take up room.

A Marshall, Super Pet or Kaytee have removable shelves that allow you to put hammocks all over instead of shelving which I find become poop magnets for those who are not great at using the box in the first place.

Do not buy the Ferret Nation 181. It is not worth the money. I only would purchase one if I was adding another story on my Ferret Nation 182.

Also considered used. Check with shelters and Craigslist for ferrets who already come with a cage.