There is not really a difference at least to my eye. The Marshall Estate has the snap together design that the Critter Nation has while the Ferret Nation has screws. That is the only difference I see besides small differences in height,width and length.
There is also a moral difference. Marshall's is the biggest BYB of ferrets in the US and you'll find many people on here won't by their products from stores as it directly supports their mass production of ferrets.
I myself have a Marshall's Penthouse, but it came with a chinchilla I rehomed.