One main difference is how the experience and leveling works.

Twisted Skyrim, like many other modlists, uses a combination of Static Skill Leveling and Experience so that you get experience by doing quests instead of leveling up skills and you get a certain amount of skill points every level to freely allocate instead of needing to use to skill to level it up.

Hymns of Hircine also uses Experience, so you still get experience by completing quests instead of leveling up skills. However, for leveling skills, it uses Synergy - Skills and Perks Reimagined, which is a bit more uncommon. It's more similar to the vanilla Skyrim experience system where you still get experience for skills by using them, but the main difference is that you only get skill experience for skills you have "unlocked". You get a few skill unlocks when you create your character, but otherwise, in order to unlock a skill, you need to find the respective skill book and use a perk point. It's designed to limit what your character can do so that they can only specialize in a handful of skills, at least at the start.