Da Vinchi or Nero Bride?

It depends on what you need more.

Nero Bride is a hybrid DPS/Support who, from my experiences with her, works well when paired with another DPS and a full support type (Waver/Tamamo/Merlin) or as a back-up DPS on the Backline. I rarely use her as my primary attacker, especially since I already have na NP2 Saberlot.

Da Vinci is more NP spammer (less damage though due to an AoE NP and her Caster class.) who works well for farming or wave clearing. Good for those quests where you have more than 3 enemies in one round but not so much against a boss unless she has a class advantage.

I would say go for Nero since she’ll get a np boost this year and is really good.

Overall Nero is better, but Da Vinci has more synergy with arts setups so I'd say go with her.

Both are good for your Art teams, however one is a selfish DPS unit and one can be a DPS and teams semi-supp. Before we continue, pretty much you've covered your Saber and Caster roles. So every choices is optional.

I have been running Art teams for more than a years now and i prefer Nero Bride. DPS with utility kit servant like Nero Bride have good synergies with almost any teams and can also work amazingly on your Art teams.
The explanations, because both is your waifu then i'm going do desribe it based on their gameplay qualities.

- Leonardo Davinci, she's a selfish Caster. Strong Wave clearer, durable Caster and farmer. Her NP can hit pretty hard and you can also spam it because her NP gain is pretty good. Of course DPS Caster can't support your party, because she's a DPS.
Not every Caster on this game is support like Hans, Merlin or Waver. You've Liz and Marie as your AOE Caster.

- Nero Bride, a team semi-supp and can also act as your main DPS. She's not a selfish DPS and she can even gave the buffs to her teammates. Both Leonardo and Nero Bride NP gain is also good.
Leonardo only know how to hit hard and spam her NP, Nero Bride can also do that and she can also support her teams as well.
Spammable NP + Def down and can also act as your back up DPS, not a selfish unit. She's a unit that you can bring almost everywhere, for daily NP spam 1-3 turns clear just use her targetable NP gain Up to any of your desired Art servants.

Because both are your waifus, Bride first after then roll for Davinci. I made this post because i've some experiences myself of using Art teams for a pretty long times.
Art teams is also my main teams and they're the most resilient teams that i have used on this game.

Bonus :
One of my favorite Art teams so far, friends Saberlot + Helena + Hans + Gilgamesh Caster. Mozart, Chloe and OG Nero on the backline.
- Medea ( Knights Dignity ) + Hans ( 2030 ) + Waver/Merlin ( 2030 ). Don't have any Saberlot so Medea is my Saberlot for now. I think this is one of my favorite and the weirdest Art team for now.
- Li Shuwen + friends Tamamo/Waver + Gilgamesh Caster.
- Chloe + Nero Bride + Hans.
- Friends Nero Bride + Gilgamesh Caster + Hans + Li Shuwen.
- Nuclear Art spam Crit team, Robin Hood/Euryale + Gilgamesh Caster + Friends Waver + Mozart.
- Mashu + Hans + friends Merlin, auto-win trump card if everythings failed.

Nerofest Grand finale 2018, Hans + Helena + Gilgamesh + Assassin Shiki + Mashu + OG Nero. First try and win, no CS and no SQ spent.
Edmond Dantes 7-2 final Challenge Quest, Squirtoria + Mashu + Hans + D'eon + Chloe + OG Nero.
1-3 Turns or One shot Gawain/Male Saber or Zerker bosses/Neutral male bosses, Euryale + Helena + Arash + Mozart + Friends Waver.

Lots of untold Art teams, putting the teams more can cause a thing called longpost. Art teams everydays.