Sparkle or Bronya - who is better?

TL;DR Who is better between Sparkle and Bronya comes down to the damage dealer of your choice and the specific setup you're using. To better understand the benefits and drawbacks of both, here is a full breakdown of what to expect from Sparkle and Bronya.

Let's compare their buffs first:

Sparkle buffs

• ATK%: 15% (45% For Quantum damage dealer),
• DMG%: 48%,
• CRIT DMG: 93% (assuming 200% CRIT DMG Sparkle - can go higher with signature easily),
• Action Boost: 50%,
• Efficacy: all buffs are active almost 100% of time (and FUA characters benefit from them fully).

Bronya Buffs

• ATK%: 55% (Normally 2/3rds uptime only),
• DMG%: 76%,
• CRIT DMG: 52% (Assuming 200% CRIT DMG Bronya - normally 2/3rds uptime only),
• Action Boost: 100%,
• Efficacy: most buffs only work on damage dealer's own action (mostly doesn't affect FUA characters).

When it comes to overall total buffs here are the standings (without Light Cones or Eidolons included):

• Sparkle (Quantum damage dealer): 186%
• Bronya: 183%
• Sparkle (Non-Quantum): 156%

But those are just the raw numbers and there are some important factors you need to keep in mind:

• Bronya cannot maintain her Ultimate buffs under most circumstances for more than 2/3's of damage dealers actions which reduces overall efficacy.
• The stat distribution of the buffs is different with Bronya offering more DMG% and Sparkle offering more CRIT DMG%. Depending on the damage dealer chosen one may prefer having higher of each of these stats, but as a general rule DMG% is generally the more desirable stats compared to CRIT DMG%.
• Sparkle only boosts her target by 50% half a turn where Bronya boosts by 100% a full turn. Something that looks like a landslide advantage for Bronya but comes with a catch because of our last reason - skill Point consumption and generation,
• Sparkle is completely skill point positive generating 0.33 skill points per action she performs but also grants a large influx of skill points at the start of combat. Bronya on the other hand is completely skill point negative at E0 without considering Light Cones (-1 skill points).

Playstyles - list

To full understand the impact of all points mentioned in the section above, it's time to compare the popular playstyles for both Bronya and Sparkle.

First let's go over the Hyperspeed setups for both:

Hyperspeed Bronya (160+ Speed)

• Consumes 1 SP every damage dealer action (-1 SP economy),
• Consumes 1 or more SP from damage dealer totaling at least -2 SP per action under most circumstances,
• Grants all buffs above with 66% ultimate uptime (under most circumstances),
• Grants current Speed to damage dealer (160 Speed),
• Carry requires 0 Speed investment (can run full DMG subs and DMG boots).

Summary: Skill point restrictive to run but not impossible. Made easier with Bronya Signature and E1 but requires careful SP management and two other SP positive team members.

Hyperspeed Sparkle (160+ Speed)

• Generates 0.33 SP every damage dealer action (+0.33 SP economy),
• Consumes 1 or more SP from the damage dealer totaling at least -0.66 SP per action under most circumstances,
• Grants 3 bonus SP at the start of battle,
• Grants all buffs above permanently,
• Grants current Speed to damage dealer (160 Speed),
• Carry requires 0 Speed investment (can run full DMG subs and DMG boots).

Summary: Skill point friendly to run, enabling almost any damage dealer to freely spam their skill without restriction. Can also support less SP friendly team mates such as Fu Xuan or E0 Huo Huo as the main healer without issue.

Now let's compare the -1 Speed setups for both:

-1 Speed Bronya (full skill spam)

• Consumes 1 SP every damage dealer action (-1 SP economy),
• Consumes 2 or more SP from the damage dealer totaling at least -3 SP per action under most circumstances,
• Grants 50% uptime on Skill buff and incredibly limited uptime of Ultimate,
• Grants double actions (2x Speed or double damage),
• Requires the carry to be 1 Speed faster than Bronya (usually 135).

Summary: This way of playing Bronya is almost impossible to run for most damage dealers for any sustained amount of cycles. Even while playing the most skill point positive supports, such as Luocha, outside of 2 exceptions: Blade and Jingliu who cheat the skill point system and consume very limited skill points of their own.

-1 Speed Bronya (Basic > Skill)

• Consumes 1 SP every damage dealer action (-1 SP economy),
• Consumes 1 and gains 1 SP from the the damage dealer by using a basic attack on the non-Bronya buffed action and the skill on the Bronya buffed action. Totalling a net consumption of -1 SP under most circumstances,
• Grants 50% uptime on Skill buff and incredibly limited uptime of Ultimate,
• Grants double actions (but not double damage) as half the actions the damage dealer is taking are standard Basic Attacks. Worth noting these are not useless and the skill is being used in the fully buffed action but it still isn't close to double damage,
• Requires the carry to be 1 Speed faster than Bronya (usually 135).

Summary: Most skill point friendly way to play Bronya and most flexible with DPS that aren't Blade or Jingliu causing her not to consume to many skill points and still benefit the DPS greatly but at the cost of overall damage.

-1 Speed Sparkle

• Generates 0.33SP every damage dealer action (+0.33 SP economy),
• Consumes 1.5 or more SP from the damage dealer totaling at least -1.17SP per action under most circumstances (-3.5 SP every 3 actions sparkle takes),
• Grants 3 bonus SP at the start of battle,
• Grants Skill boost 66% of the time a high portion of the time (> 60%).
• Grants 50% More Actions,
• Requires the carry to be 1 Speed faster than Sparkle (usually 135).

Summary: Sacrifices Sparkle buff uptime and some of her skill point economy for converting the Speed transfer like interaction into 50% more actions.

Playstyles - conclusion

To conclude our comparison here is the meat of it:

• For characters that have no (or limited) access to DMG% increases, Bronya's buffs have an edge but for characters with high DMG% who might crave CRIT DMG% Sparkle performs better,
• The best supportive option of the builds listed above is the -1 Speed Bronya (full skill spam), but it requires the carry to use 2 skills and Bronya use 1 skill every action. And this is close to impossible to achieve outside of Blade and Jingliu but for these 2 characters Bronya is still the clear best in slot,
• The second best option is Hyperspeed Bronya / Hyperspeed Sparkle. They both feature high buff uptime (from Skill and Ultimate), while also boosting the action acceleration of the damage dealer and enabling him to use ATK% (or HP%) boots and focus his sub-stats away from Speed.Both setups are very close in performance and which is stronger numerically will depend on the damage dealer, sub-stats and Light Cones being used. But it's without a question that Hyperspeed Sparkle is substantially more skill point friendly than Hyperspeed Bronya, enabling the smooth use of sustains and supports that can't generate as many SP,
• Next in performance is the -1 Speed Sparkle build that can still sustain the skill spam playstyle of the damage dealer albeit with much more difficulty compared to the Hyperspeed Sparkle while still offering reasonable uptime on buffs and the 50% action multiplier,
• Last in performance is the Skill -> Basic Bronya that asks the dps to use 1 basic and then 1 skill after Bronya boosts them cutting their damage in many cases substantially (also with the cost of having low uptime on Bronya's buffs). This composition is the most SP friendly of all Bronya's options but lacks the buff uptime and skill usage of all 4 options putting it in last place.

You can see the comparison visualized here with a simulation of how each of the different builds impacts an endgame E0 S1 Imbibitor Lunae with Sparkle or Bronya as their only support over 8 actions. Observe the impact each build has on damage but also net skill points consumed by the combo (Sparkle's technique SP and Bronya Signature SP are included):

Disclaimer! We know that E0 Imbibitor Lunae isn't the best when played with Bronya, but we decided to use him as the example due to his excessive skill point consumption - which allows us to better showcase the difference between the setups.

Summary

Bronya is best for Blade and Jingliu as she can edge out Sparkle when both are being played in the Hyperspeed setup (when skill points aren't a concern) but Sparkle is close to (or even beats) Bronya in cases when CRIT DMG is valued over DMG% or when the damage dealer belongs to the Quantum element.

Still, the flexibility of Sparkle's SP positive nature and her ability to boost the damage dealer's output outside of their own turn (which is very important for FUA characters like Jing Yuan, Dr. Ratio or Clara) pushes her usefulness and makes her the more universal support compared to Bronya - and by a good amount despite her maximum ceiling being lower for some characters.