Good question, I only have Arjuna alter and he hits hard thanks to his NP effect.

It depends who you Like more, though in many case, Arjuna Alter seems can do better than Amakusa. But if you're still not sure who to roll, then don't roll at all. Wait until you see a servant you really want to roll without thinking.

The same question, only Dantès is added to these two

The same answer, only Dantes is added to these two.

Sometimes the best thing to do is do nothing, or in this, roll nothing. If you don't know who to roll, then don't roll at all. Simple as that.

Arthur appeared there, whom I chose without hesitation. I just wanted to inquire about the supplement to it.

Then imo, choose Arthur, forget anyone else. They all will have return banner one day, maybe, but atleast you'll get your most wanted servant.

As someone who have both, I can say Arjuna will definitely help you more than Amakusa. Arjuna has better Buster crits and NP1 damage, while also being pretty good at farming, and if you have powerful supporters like Merlin, Oberon or Koyanskaya, you can easily dish out some crazy damage. He's basically like a cheat code. Post buffs Amakusa is also extremely good, but he's more like niche tanky DPS that you'll most likely bring in a difficult stall battles, or if the enemy is spamming buffs every few turns. I guess you can also use him as a semi-support, but idk if anyone even use him that way. Overall, both are amazing units, so pick whoever you like the most