Salter Vs Altera

Gonna leave this one to ruzeron since he's the altera expert. My personal opinion though, if you have other servants to develop first develop them before altera and work on altera eventually

Their NP damage is nearly identical with Salter having a slight edge at low NP levels and doing slightly less damage at high NP levels so from the perspective of K scope 3 turn farming they are essentially identical. On the other hand when it comes to normals Altera wins hands down with some of the hardest hitting normals in the game thanks to a massive attack stat and 30% attack buff on a short cooldown.

Basically if you have a proper farming set up they perform essentially exactly the same, if not Altera will be a fair bit better but I don't think its such a big difference to prioritize it over other things if you need to level other servants too.