Gawain or alter

Saber Alter deals way more NP damage than Gawain. Her NP multiplier is 450% at NP1 which is an oddity, that makes her do comparable damage with NP1 OG Saber despite the stat and charisma difference. Gawain at NP1 has 300% multiplier, 5* Artoria has 400% after NP interlude.

To give you an idea. Gawain at NP1 deals 15k with his Excalibur Galatine (19k under sunlight), Saber Alter deals 29k with Excalibur Morgan (NP1) anytime. That's basically the same amount as 5* Artoria and it's the highest AOE NP damage of the Saber Class...Unless you count Siegfried against Dragons.

Stick to Mordred, raise her to lvl 90 and her skill as well.
They do the same jobs, wave clearing and farming.
Mordred have NP similiar damage to Salter but tougher and have her own NP battery.
NP damage is not the same but it's just very close.
Plus, Mordred have a bonus as an anti-Artorias ( Siegfried is an anti-Artorias as well, because most Artorias is a dragon ).

In terms of general performance, Salter and Mordred is technically equall.
For more specific and tougher fights, Mordred wins.

Another case, if your Mordred has been maxed you can raise Salter because you will farm everyday using both of Gold saber.
Overall performance, Salter is far stronger than Gawain.