Cu Prototype

You already have good lancers. While Proto Cu deals more damage, he has a niche damage. Even if you like him, I would suggest not raising him for now and focus on other servants like Cu FSN.

I won't prioritize him since he doesn't have his rank up yet, but later, he'll be a good crit monster. Overall, he is a better FSN Cu in team.

Personally I chose Cu Proto over Diarmuid. He has better survival skill in protection from arrows, and his NP has a chance to cause death.
I have a slight bias since Proto was my first NP5 lancer. His NP death trait has saved my bacon several times.