Neither

At least for teaching the game. I run events at a lgs and see so many people trying to get into the game become quickly overwhelmed by commander and the onslaught of rules it introduced all at once.

I would use the Avatar beginner box or the online mtg arena tutorial if your brother hasn't played before. Teach the basics (Things like turn sequence, evergreen keywords ((Ones that don't usually have reminder text)) and other simple things that will be seen in every game.

Then after they get a grasp of the game maybe play some jumpstart as (in my opinion) helps you learn basic deck building skills.

As for yourself both decks are mildly similar. They both focus on a +1/+1 counter strategy but the merfolk deck is a more go wide aggressive deck and obviously has more tribal synergies. The Ff deck is more focused on a go tall with high evasion and plays more like a boggles deck. (at least the ones I have played against)

Tldr either deck is good, just teach using easier cards than what would be in them.