Nero bride or Musashi?

Based on your preference. Nero bride is a support saber and to use her offensively you need to use all her skills on her. She has weak survivability due having no evade and such
Musashi in the meantime is a buster gorilla having 3 buster cards, has average to nice NP gain due to her first skill, an invincible Pierce Mana burst, and an invincibility.
It's honest up to you, but I'll go with Musashi honestly with the existence of merlin

To add something to it, it does depend on the playstyle you prefer as well. Nero bride would be excellent and giving your main arts dps the supportive tools he or she would need to keep up their np spam while being able to deal decent supplementary damage herself.

Musashi on the other hand is pure buster dps with potential to crit in memelin comps

Rare servants can be compared if you won't make them efficient in your team building thoughts.
Musashi will be one best ST buster servant with excellent NPBB chain.

However, in my opinion there are three drawbacks for her, or I may say, the reason why Musashi won't be highly ranked in JP recently could be because of.....
1. Too short skill lasting but high cool-down.
2. NP gain is great but only 1 art card, rely on 1st skill
3. Basically no star adsorption and critical-up skills.

Therefore, reason why Musashi + dual Merlin is amazing since the presence of dual Merlin almost covered everything.
No star adsorption? No problem from 4060 and Merlins' NPs. Enough Star!
No critical-up skill? No problem, dual Merlin can provide 200% critical up in one round.
Short of Art card? No problem, dual Merlin's have 40% NP for team in 5 rounds, 6 art cards which is high potential for art chain, and NP. Oh BTW, Waver where are you going? I prepared Combat Uniform.
Skill has short lasting but high cooldown? No problem, dual Merlin's 100% Buster-up made Musashi's normal 3 Buster cards even better than Berserkers against any classes except archers and rulers.
Need to wait for an perfect round of Musashi's critical NPBB but enemy would charged their NP? No problem, dual Merlin's team Invincibility can at least against 3 NPs in 8 rounds. Also their HP recover from NP are also OP.

All in all, an possible damaging of Musashi's critical NP(lv1)BB with all buff on from both her and dual Merlins versus lancers =
~220,000+310,000+370,000+56,000
Can even some archers sustain such damage?
And this doesn't even include CE's attack status and offensive effect like limited/zero over, benefits from Mystic code, etc.
Even a normal round, under dual Merlin's offensive buffs, her one Buster card can still provide 28000-85000 damage to lancer class.

I am not marketing Merlin. But the comb of Musashi+Merlin > Others since their compatibility are really amazing. It is still hard to say Nero Bride or Musashi which is more deserved to roll. If you put Musashi in art team, she would still be lacking of performance. Same thing if you want to have Okita or Bride with dual Merlin, they are also not capable to provide such amazing damage as Musashi. Right now you can consider Musashi is somehow a servant with Li Shuwen's playstyle (one round bursting, very limited beneficial for team), but Buster card can provide way more damage than Art card and Merlin's supportive role is also quite different to Tamamo.

Nero Bride on the other hand, is one best DPS/support mixed. Her three skills are amazing for either herself or team. Then she is one most versatile offensive servant. She can be typically Hitter in an Art Saber Team, and her 1st skill and 3rd skill are amazing for Tamamo/Waver or Hans because normally an art team's playstyle is long-tough relying on art chain for DPS's NP damaging. Or maybe in Art-critical team then she will also do the NP spam work. It is also decent to put her as support like Vlad+Bride+Tamamo team (Vlad NP Spam), or teamed with Mashu/Jeanne/d'Eon for more tough job. It is quite good for challenge quest when facing multi-class enemies.