Either would be fine for a 6 month old.
There are some pretty big differences, one is size, the Feisty Ferret is much smaller then a double Critter Nation, (14 cubic feet vs 24 cubic feet) here is a photo (not mine, and both have been converted to wood shelves) that shows a double Ferret Nation (same size as a critter nation) next to a Feisty Ferret
Another difference is the bars, the Feisty Ferret has 1" mostly vertical bars where as the Critter Nation has 1/2" mostly horizontal bars.
Also the CN both doors open allowing easy cleaning, redecorating, and catching the chin, the FF only one door on each level opens.
The FF is also more prone to rust and bars breaking, most people say they last less then 5 years before needing to be replaced, others only had it a month or so before bars started snapping on it. I have had my Ferret nation (same cage but 1" vertical bars instead of the CN 1/2" horizontal) for around 10 years and it's still in excellent condition.
The FF also has mesh floors that need to be covered to avoid foot and leg injury, the CN comes with plastic pans for the two floors and two shelves that need to be covered or replaced to prevent chewing them.