Stormworld is mostly the same game rules but with new fantasy content. Elves, gnomes, frost giants, rivers of magma, talking helmets, dragon mounts, that sort of thing. Basically a different setting for the game, filled with "traditional fantasy" content.
I love Stormworld. Since each of us are working alone on our projects, the modder's skills determine the direction of the mod. Stormworld offers amazing visuals and excellently thought out races. The wiki alone offers hours of interesting lore. The rivermod is truly a work of art. The reliquary offers a chest full of items that are able to actually feel as if they are magical. It's a great place to start for players that like FE, but demand a traditional fantasy setting for their games.
But there are others that feel FE is lacking depth. Master's Affliction is designed for expert 4x gamers. It offers a better RPG progression for your characters. It makes the world a seriously dangerous foe. City leveling has a massive level of specialization. Leveling cities high is as powerful as city spam, making each a viable choice. It increases the types of viable units in the game. FE has two unit types: Heroes and Troops. Master's Affliction has five: Sovereigns, Heroes, Troops, Summons, Monsters. Sovs are immensely powerful from turn one. The mod boasts a more robust faction and Sov creation. Troops have a wider variety of tactics, derived from resource strategy. Summons are extremely powerful, but admittedly I need to add Goetia to really flesh this area out. Monsters like Ogres and Trolls now play a vital role in army supplementation. There are a huge list of changes to areas that I felt were under-realized by the devs. Master's Affliction seeks to give the expert players a new challenge and grants the wishes of those that say the vanilla version lacks depth.
As you see, the mods are aimed at two types of people. I suspect most will begin with Stormworld, as it is the most familiar to them. But eventually even with all those pretty centaur models, players will come to contract the master's affliction for challenge.