For combat, a thrall will always be more powerful in Conan Exiles. Not only can you find thralls that can be leveled with superior combat stats, but you'll also be able to equip them with high-quality gear to add to their damage output and survivability. While "better" is an objective question, T4-named thralls are the best-in-game companions. However, animal followers take much less effort to procure and no effort to equip (because they currently cannot be equipped with gear). Their stats are still great and arguably a better trade for your time invested.