Right now they are my only 4* casters and i have no one better than them (F2P and E-rank Luck) so who should I level up first? I know that Helena has some decent Support abilities but Nito can help me farm mats easier so I'm a bit confused here.
Can't give a definite answer without a bit more context. But I guess it depends on how far you are in the game and/or what you need more.
If you're at the point where farming is more important in order to... well... farm materials for ascension and skill enhancements, then Nitocris would be a better choice.
But if you need a combat team more for story or event progression, then Helena would be better to help support your other Servants.
I can't really speak for Helena, but the earlier you are the in story, the better Nitocris probably is, as her death will trigger much more often. In Camelot's story, it felt like my Nito was able to trigger 1/3rd of the time against a group of Enforcement Knights, which still felt like it was saving me time, since they were damage sponges and I could easily get two NP shots off in a row with her.
And obviously if you just want to farm, Nito is simply amazing at it. With daily embers you just charge her up during round one, and she practically clears the next two rounds by herself.
In my experience...and luck, nitocris could also be viable for babylonia. Even near the end when you get king hassan as support against those high hp swarm, she managed to kill 1-2 every np. I also brought a cursed arm hassan with bond ce which helped though.
If you Have Hans/Waver/CasGil/Tamamo raise Nitocris, if you have none of the above raise Helena.
Nitocris is an amazing farmer and against regular assassin enemies she can abuse the instant death mechanic. And she is very good at that job, but she is no Sanzang, she is good at farming and that's it. But she will prove very useful.
Helena is worse than every other support except Edison. Her 1st skill at lvl 10 charges the party's NP gauge, which is good but it has 7 turns of cooldown. Her 2nd skill only produces 5 stars regardless of lvl and has high cooldown. Her 3rd skill barely has any difference between lvl 1 (15%) and lvl 10 (20%) but it is the only Quick buff in a Caster until Skadi. Her NP hits like wet paper but the good thing about it is the debuffs it deals to the enemy, it drops defense, attack and debuff resistance which is is very usefup in a stall battle.
Don't get me wrong, she is a decent support, but she is too expensive to be just decent while Andersen takes almost nothing to be better than her.
Alexander also has AOE 3 turns Quick Up, after Strenghtening quest. But he's a Rider, will rob your main dmg dealer for their stars very frequently. Which you probably don't want, he's AOE 3*s with bad hitcounts.
Also, by Agharta we get Wu Zetian, the 4* Assassin chinese naked loli, with a AOE Quick up who does not include herself, sadly.
I know, that is why i said Quick buff in a Caster. Alex and Nyanta have AoE quick buffs but they have vey high star absorption and basically no support except Alex's Charisma.
Wu will be good but she is still unreleased and is an Assassin, she requires different mats.
Osakabehime will have AoE Quick buffs but tied to her NP, and she is a 5 star that will only be added in october.
If you're new to the game just focus on Nito first, she's easy to use and pretty much a straight forward servant.
If you have play this game for a pretty long time or even a seasoned player, Helena is a good friend of Hans. She's a general support like Waver and i would say she's one of the most devastating short burst support member out there yet. Be it a Crit teams, farming, boss killing wingwoman/support as long it is not a longer battle or something like that she's good, but i'm not recommend to raise her earlier if you're new to the game.
Her only weakness is her long CD, she's not really suitable for longer battle but still usable.
You'll gonna raise both of them anyway when you've enough resources.
