Someone who plays Fiora explain to me what differences do her and Jax fulfill on a team comp

Jax scales on stats.

Fiora scales on stupidity.

Irelia scales on both.

Fiora for 1v9, jax for 1v2.

I am by no means an actually Fiora main (though I have been playing her a lot lately), and I don't really play Jax at all, but this is how I see it:

• Jax is a better splitpusher, and lategame a better 1v1er. He takes towers a lot faster and eventually just outscales almost all opponents in a 1v1. Fiora can also beat most people 1v1 lategame, but it is more skill dependent.
• Fiora can contribute more to a teamfight IMO. She doesn't require you to really go in deep, and there's a really strong teamfight pattern she can take of "ult their frontline, demolish them with your %HP true damage, then charge the rest of their team refreshed by the heal."

They're both duelists, but I personally find Fiora harder to deal with, no matter who I'm playing.

Why would anyone pick Trundle in lane over Fiora, especially now that their ults share the same cooldown? She's a better duelist, team-fighter, tank shredder and, with his latest nerfs, now she deals more damage to towers.