Scathach or Ereshkigal?

Whats your team comp looking like? If you have more quick servants, Scathach might be better for you. Depends on who you have to build around them.

Ereshkigal is a AoE Buster Servant while Scathach is a ST Quick Servant

both have a the same cardset being 2 Quick, 1 Arts 2 Buster, they also have a 1 Turn survival skill
(Evade for Scathach which can also give her star absorb and crit dmg+)
(Invincibility for Ereshkigal which chance for other deffensive buffs like debuff resist, buff removal immunity, death immunity)

Scathach has a Dmg up against undead and divine + a targetable quick buff while Ereshkigal has a strong buster buff for herself that aditionally gives her up to 50% NP charge and a party wide defensive buff that combined with her np also increases attack)

so in the end it depends on what you need/want.

One is AOE Buster with 50% (max) charge bar, buster up, her NP gives some party buffs if her 3rd skill is active. And has invulnerability.

Other is a boss killer (specially undead and divine ones), God Slayer+Primeval Rune cause obscene dmg if combined with her NP. Sadly, with that shit NP gain, the introduction of break HP bars will hurt her pretty bad...but she has guatanteed stun and an evade.

You already have a Buster ST (OC dmg+ vs Evil), a Buster AOE (likely NP5, so will cause more dmg than Eresh NP1, and both have 1-turn buster ups and invulnerability) and a Quick ST (80% chances of stun at 100% OC, but is sorta weak).

I think you are sorta good of Lancers (leveled your Cu's, bro?), yet, you should choose Scat.
Eresh sort of has her functions covered by NP5 Jalter Lily, aside from the minor NP charge who does not progress by level. Medusa Lily NP causes very low dmg, and her kit cover different functions.

- If you want to build another ''walking fortress'' roll for Ereshkigal. Both option won't dissapoint you, there are Jalter Loli that can cover AOE roles. Skipping Scat is also fine because you already have 3 ST Lancer and they're strong enough to cover that role.

- If you want to improve your ST DPS potential roll for Scathach. You already have OG Chu lvl 80 anyways, use him as your soloist and Scat as your main ST DPS. Even Vlad III can be used as your Buster Crit lancer.

- Gameplay wise, skipping Scat and Eresh banner is also fine and i assume you're debating yourself who to choose between Scat and Eresh. Waifu wise, don't skip it, if both of them is your waifu.
You can also toss a coin, to decide who to roll next.

Gameplay wise again, all of this seems optional because you already covered Lancer ST AOE roles. For balancing some stuff, i think Ereshkigal can help your Jalter Loli cover AOE roles.
So it would be 3 ST Gold Lancer and 2 AOE Gold Lancer servants, it seems pretty fair.
If you roll for Scathach, it would be 4 ST Gold Lancers and 1 AOE Gold Lancer servant ( Jalter Loli gets overworked LOL ).
Btw, i have Li Shuwen as my main ST Lancer and OG Chu lvl 90 as my Last Man Standing or second DPS. Plus Taiga Fujimura as my Buster Crit Lancer.
For AOE Lancer, i have Jalter Loli and Elizabeth as my second AOE/back up DPS Lancer.

Bonus : Give Jalter Loli a break LOL XD and all of your lancers have great solo potential, including Medusa Loli ( she even can destroy almost all male enemies if combined with Euryale ).
Don't forget to make some videos while doing solos.

Honestly it depends on the servant line up you plane to have. Both Scathach and Ereshkigal are great especially if you pair friend support Merlin with Ereshkigal for a buster team since even though Ereshkigal has 1 arts she can basically spam her NP and she provides buffs for the entire party. However, Scathach is a awesome single target quick star team where in the JP FGO the meta is quick teams now thanks to Scathach caster. Both are very good and everyone else has provided great advice but it really matters on what team you want to try and dedicate yourself to either being Buster with friend support Merlin or Quick with future friend support Scathach caster. If you are not caring about teams and just want to have a servant for fun then I say Ereshkigal since she has a great personality and her rate up is rare.