Helena vs Edison

Edison was considered one of the worst 4 star servants since he offers very little to the team due to his limited niche. Once he received his strengthening quest which added a 3 turn ATK debuff on his NP on top of his NP/Skill stun however, he became a lot more useful though that won't be for a while now.

Helena can do a lot of things from charge her team's np gauge, provide a party wide buff to all card types, generate stars and deal decent damage with her NP (should RNG be in your favor with her 2nd skill) but is held back by her skills having large cooldowns despite having merely decent values.

Tamamo no Mae is the best servant since she can help charge their NP gauge and reduce cooldowns.

Helena is a rare farming team support. Pretty sure all her skills is geared toward farming. Party wide np charge, enabling everyone who has kaleidoscope to immediately use their np, self (missable) np damage up for her AoE np and universal party wide card type effectiveness up to support everyone's np. Pretty good if I may say so. In other team setups, her long cooldown is a big hindrance, and honestly, I dare to say that the 1* and 2* casters perform better than her.

Edison is a more specialized support. His biggest niche is his third skill (which isn't available yet AFAIK) which gives a servant free +200% overcharge when using their np (similar to Demonic Bodhisattva CE). He is perfect for servants who have useful overcharge effects or servants whose np damage scales with overcharge (Robin, Arash, etc.). On a top of that, later on, his np provides attack debuff, which is always useful for stall team. Though, right now he has none of those niches, so he is pretty useless.

If a farming servant is what you want, Helena.
Also try levelling Kiyohime up, she is one of the best farming servants out there.