CasGil or Tamamo?

There are a lot of questions that factor into making a decision like this like:
1. What is your Caster champion pool like?
2. Do you possess any offensive Casters?
3. Do you prefer running Caster-only lineups or you include an offensive servant from another class that has reliable damage?
4. Are you looking for faster clear speed or you're ok with grinding a node with Support-only Casters?

One major problem I have with casters is that whenever you're running an all-Caster lineup against a high-level Assassin node with beefy Servants or bosses, its a grind. This is mostly due to a lack of high dmg ST NPs for the class. Most times I have to rely on Da Vinci or Medea for dmg, and even then its not a fast run-through. I can count the times I've seen Medeas NP dmg go over 45k on one hand, and still have 3 fingers left over, and Da Vinci's AOE is great for wave-clearing, but not so much for bosses or Servants with HP over 100k. I do have Sanzang, but shes not really a priority for me at the moment.
CasGil is a good Servant to pick up if you're looking for a Caster with a little more "oomph", but if you have maxed Support Wavers or Merlins, then whatever Servant fills the frontline, its already going to be a tough lineup to beat. The only things I can think of that ruins a lineup like that is if the synergy is broken via a ST or AOE NP nuke that just obliterates one or all of the frontline, but otherwise, you'll make the enemy bleed for every HP point they take, and heal it back up with Tamamo, but its going to take a lot of turns.

TL;DR
Feel free to pick up CasGil if you're looking for a reliable AOE Caster.
If you don't mind your battles taking a little longer, then it's probably not worth devoting the resources and materials to bring CasGil up to par. as he requires a lot of pages (44) and lanterns (60).

caster gil is not a bad servant.
i even use him as the main damage dealer when facing multiple small assassin, or as the secondary dps in case of big one.
my standard comp in those case is casgil, tama and sanzang. the basic idea of this team is to spam np, and boy you can do it easly.
i was able (tnx to gil star) to cast his np 3 consecutive time, and the same goes for sanzang.
true you dont have lot of buff for sanzang np, but this comp is more about quantity over quality.

CasGil is semi-support. He can deal damage and works best on arts crit teams with passive stargen. Since arts cards don’t drop stars, his active stargen boost is mostly nil unless you’re using an arts assassin or archer like Chloe, Robin, Kerry, Serenity, or Shuten.

Tamamo can be offensive support just for her Fox’s Wedding and NP but her other two skills are much more defensive (at least until her rank up) and they make her a difficult pick for fast arts teams. Since her battery lives on her NP you need to be thinking a turn or two ahead of time + counting cards and cooldowns for her to really be effective because she can’t facilitate instant NPs like Waver, Merlin, Helena, or even Mash.

Tamamo also has no stargen to speak of so running her with CasGil is a wonky comp to begin with without 2030 and Saberlot or an assassin. Merlin is a much better pick since he’s basically a faster Tamamo, can stack charisma with CasGil, and provides stars. Of course, you could use both, but you’d have no room for CasGil and you’d run a weird semi-stall np-spamming arts crit comp...it’s weirdly effective for servants with a high starweight (Robin, Ozy, Mebd, Cleo, etc.) but there’s no room for CasGil.

If you want to use CasGil, you need to use him in a team that appreciates his boosts (so, arts crit) but can both supply stars and heals/mitigate damage. Merlin is a good partner, like I said, and so is Hans. Jeanne and Mash could work with a rider as the DPS but they’d compete with a Saber for stars (unless that saber is Lancelot). Mash also doesn’t have a heal, so that’s a little rough.

Unless you’re supporting pretty self-sufficient servants that tend to drop a good number of stars on their brave chains like Kuro, Ozy, Gil, or Shuten, you’re better off using 2 supports and a single attack if you want to use CasGil.

CasGil’s problem is that he supports active stargen and arts cards...which don’t produce stars. So most arts crit teams use passive stargen...so his active stargen buff is kind of awkward.

He’s like Bride in that weird semi-support space...someone else can do what he does better but his draw is that everything is in one package. So you need to capitalize on all of his buffs for him to be a better pick than Tamamo.

That’ll change once Sherlock comes. Sherlock + CasGil is a great core. Sherlock produces a lot of stars, has the star weight to grab them, and is a crit monster after his NP goes off. Throw a Hans, Saberlot, Kerry, Nursery Rhyme, or Squirtoria in there and have fun.

Casgil is better than Tamamo for arts-crit teams. Because, you know, he transforms multi-hit arts cards into quick cards, basically. Tamamo needs a 2030 CE to supply art-crit as well as CasGil.

With arts-crits he can kinda spamm his NP, allowing for stackings of def-up/def-down. Very useful for hard fights with assassins with many HP.

Since Tamamo arts buff is targeted, not AOE, Casgil sometimes is also a better option for certain formations, but be sure to max his skill, as the cooldown of 2 of them is bad as hell. And yes, his skills are costly.

I consider him a costly investment, with huge benefits. Tied with Helena as the best 4* support. With maxed skills, and in the right team, he outmatches some 5*s.