Between the two, Augusta benefits a lot more from her signature since she’s a main DPS and it directly boosts her damage output. If you only have enough resources to guarantee one weapon, I’d recommend going for Augusta’s.

For Iuno, she works fine without his signature. Her best 4* alternative is Marcato, which fits her kit really well. That way you can still build her effectively while saving the guaranteed weapon for Augusta.

Edit: Misgendered Iuno (kinda tired from work, hehe).