I was fortunate enough to pull Sanzang yesterday. But I already have a np2 Illyasviel. I will be leveling her up regardless, but in terms of skills who is prioritised and who would be better to bring into battle?
In terms of skills, Sanzang is a lot better imo. Her skills are perfect for her and bring a lot to the team, whereas Illya's are a lot more selfish. It's hard to say, but considering the sheer NP damage advantage Illya has over Sanzang at NP2, I'd say for you Illya would be better, especially against higher HP bosses.
When fighting against Assassins bosses or mobs, you need you Caster team's combined offensive, rather than accumulate buff to one single DPS (common team of DPS + dual supports), because
(1) casters has 0.9* multiplier
(2) Very rare offensive casters have star absorption skills, in other word, critical team wont easily applied for your Caster team except Alice (Nursery).
(3) Same as (2). All casters are generally wont generate high amount of stars like assassins.
Therefore, although NP2 Illya has better NP damage than Xuanzang, overall Xuanzang wins due to her golden-ranked 3rd skill, and decent 2nd skill.
Even not in Caster team fighting against assassin boss, Xuanzang is more versatile than Illya in other teams. For instance, his 2nd-3rd skill made her good with Berserkers.
Or some specific servants like Tamamo Lancer and Jeanne.
Her AAAe NP gain is 2nd best just slighly below Liz Caster, which make her spam NP faster than Illya.
Illya's function is too narrowed.
Illya is a tactical nuke, but has major weaknesses that you need to build around. Sanzang doesn’t hit as hard with her NP but she can spam it, and she is far more self sufficient. She can slot into most teams and contribute. Between them, Sanzang is much more consistent damage and Illya good if you need an Assassin or Zerker dead ASAFP. I have both and I find they actually work really well together. Sanzang’s third skill really helps out Illya and with luck allows you to use her NP again MUCH faster if you get a NP A A brave chain. Pretty much anything that’s not a Rider or Ruler is going to torn apart by a Illya, Sanzang, Illya NP rotation. In fact my favorite team at the moment is these two and Nitocris.
