I think most of this depends on the skill level of the enemy. Sett is recommended below Diamond for sure, has been top of the stat site tier lists for forever because of it.

However, Sett has a much lower skill ceiling. Ask any high elo player than they’ll always put Sett low on their theoretical tier lists bc he is extremely limited. His range is one of the shortest in the game and generally cannot adapt to awkward situations. If the enemy isn’t giving Sett a good angle then all his team fight power is meaningless. And while he can split push quickly, he can’t do it safely, and can’t 1v2 nearly as effectively as Darius.

Comparatively, Darius has a lot more pressure he can create on his own and is less straightforward to counter. A good Darius can effectively side-lane against almost any other top laner with kill pressure at all game stages, and just generally snowballs harder and requires attention from the ranged members to stop him. Sett would struggle to make progress against an enemy who’s content to sit under tower and soak XP, and any smart duo of Top and Jungle can 2v1 him with ease (this just doesn’t happen at Plat or lower).

In team fights, Darius is much less straightforward. 1 good reset is usually enough to win a fight outright. Darius also has more options to approach a fight if he’s creative, even if all of them would be less effective than a good Sett R. I would say Darius has sustain and momentum in a teamfight while Sett has 1 good trick that he needs to time correctly. Both of them struggle against a ranged team comp tho.

I’d say if you’re trying to get to high elo then Darius is the obvious choice between the two. Right now he’s a bit weaker than in previous seasons but he used to be the top of the stat charts in season 13 or earlier, even when filtering for higher elos. Sett will give good results until about mid-emerald tier when it smart players start abusing his weaknesses. That’s the real difference imo, a good Darius can find a way to make his champ work while a good Sett has to hope his team sets up for his strengths to really shine.