For farming purposes Halloween Eli's damage is good enough if you have her at NP5, but so is Siegfried's as long as you have a Waver to buff him. Bear in mind that salter is on Merlin's banner too, so if you don't have him and want him it could be a good opportunity to go for both. For overall farming purposes, I would go Salter (if you aren't rolling for her on the banner), in my experience Nito isn't as reliable as I'd like her to be unless she's NP>1. Also, Salter's story locked but Nito can spook you. I am a bit biased though, I want Salter and I've been spooked by Nito three times.
Saber Alter or Nitocris?
According to my Clairvoyance EX , in September will be released Merlin's second banner and a free SR ticket. There is a lot of time left until then, but I want to ask who I should choose. Nitocris lacks damage, but AoE instant death is very powerful for farming. Saber Alter deals a lot of damage but lacks other aspects such as survival or NP gain. My only AOE Saber is Sumanai and my only AOE Caster is Elizabeth Bathory.
I've fully grailed my baber alter and have never regretted it, for both waifu reasons and for game play reasons. Yeah her np gain ain't has high as most, but it's still pretty good, especially if you you chain her arts card after one of her cards like qa, ba, or aa. Plus her np extra damage is damn good. If you have Santa baber alter, they're basically the same minus the skills, so she's a good test run to see if still can't decide.
Also along with the merlin banner later in the year, baber alter will be featured in the 2nd shinjuku banner in about a month and a half if you feel like saving quartz now. Get her then, and it won't matter.
I was completely disappointed by Nitocris when I finally got her. I max leveled her Egyptian Magecraft, which means I have a 150% chance of instantly killing enemies. Yet even some bronze enemies still resist this mechanic regularly. If you're lucky you might kill a silver enemy once in a while. The thing is, I usually don't need instant death to kill bronze enemies with their typical 10k hp. Any other NP would suffice for that.
On top of that, Nitocris' NP is pretty weak so any enemies which resist instant death are bound to be still standing afterwards. This makes her pretty useless for farming the 40 AP dailies.
To sum this up, Nitocris is the most overrated servant on the 4* tier list IMO. Go for Salter.
I've got Nito myself at Np1 and maxed skills and with Black Grail. Her instant death mechanic isn't reliable enough and her damage isn't enough to clear 40AP dailies. However she is useful in event quests where the wave is all less than ~15,000HP as she can one shot them without the instant death proc and holding an event CE. But for 40AP dailies and waves with HP > 20,000, she in all honesty worse than Arash/Spartacus with MLB Imaginary element / Kaleidoscope as they can also get to 100% NP. Both these guys do around 25,000 damage without extra buffs and are who I take on my 40 AP dailies, especially QP farming.
As for Salter, I can't say much but I do want her myself for waifu/bustermeme reasons.
If you don't have a kscope or MLB imaginary element, Nito is the best thing you have for one button AOE clear. But if you do have these CE's, Arash/Spartacus is much more reliable and usable in more scenarios, ahem QP farming.