Ah yes, one of the most highly debated and toxic topics you'll ever see in the Naruto/Boruto fandom. It's like the ninja equivalent of Goku vs Superman, except this time one of them has tiddies and the other is basically ninja Freiza.

Now if these two were to have a fight, I would personally bet on Momoshiki.

I believe Momoshiki is stronger, and I'll explain how a vs matchup would go between these two. I'll compare them under these following stats:

1. Attack potency (AP)
2. Destructive capacity (DC)
3. Speed
4. Hax/Arsenal
5. Intelligence and overall skills
6. Durability and longevity

Note that AP and DC are two different stats entirely for a reason. AP is the maximum potential output a fighter can use in an attack to take down an opponent who already has a fixed scale. DC on the other hand is the collateral damage that they cause when the attack lands on the opponent.

For example, Majin Vegeta scales above Teen Gohan and Super Perfect Cell, making his AP multi solar-system level. His DC on the other hand would be planetary level when he uses his strongest attack, because the epicenter of the attack was focused on the target, not his surroundings.

Now that we've cleared that, let's begin.

Attack potency and Destructive capacity:

So we'll start with Kaguya's AP. After absorbing the chakra from the entire planet, Kaguya was able to create a dimension collapsing truth seeking ball, which was intended to destroy the entire dimension Naruto, Sasuke, Sakura and Kakashi were in.

Considering the fact that each of her dimensions contained a star and a few planets in it, it's safe to say that her AP should comfortably scale to Solar System level+.

Base Momoshiki in the novelization was stated to be able to destroy entire stars and harvest the dead star and it's planets into chakra fruits. His power could even extend across different space-times.

And yes, this is canon in case you're wondering. This actually took place off screen:

If you still don't consider it as canon, there's even more proof that Momoshiki's AP is multi solar-system level+.

Remember when I said that Kaguya can create a dimension collapsing TSB with ALL of Earth's chakra?

Momoshiki can also perform the same feat IN BASE.

And yes, it's a truth seeker ball, not a bijudama like some Kaguya wankers would say. You can see the five chakra natures surrounding it, and a TSB is created by blending all five chakra natures with yin-yang and light-shadow release:

Some people downplay this feat because it barely destroyed Konoha. But that's where AP and DC come in handy here.

Naruto took the full brunt of the explosion with his Kurama avatar and six paths chakra, which means that the collateral damage was enough to level a country:

So while the attack had an AP of solar system level, the DC was country level.

Saying that Momoshiki is country level is like saying that Moro is galaxy level, simply because one attack of his can destroy a galaxy.

Which is obviously not true, since Moro has been proven to be equal to MUI Goku who's low multiversal.

Like I said, AP and DC are two entirely different things, and this causes misinterpretations when it comes to scaling characters.

And it's kind of obvious that fused Momoshiki would be even stronger, because he has Kinshiki AND 50% of Naruto's chakra stacked onto his base form.

So Momo takes the win here.

Momoshiki: 1

Kaguya: 0

Speed:

Kaguya was relative to boil release EOS Naruto, who was able to land clean hits on her:

Enraged Naruto could actually outspeed her and even managed to chop off her hand:

Momoshiki on the other hand was able to take on an older, stronger version of Naruto as well as Sasuke when fused:

And it has been stated before several times that training boosts your chakra reserves and overall stats:

While their chakra reserves were depleted, that wouldn't really change their overall physical stats, but it would affect other factors like vitality, stamina etc.

So Momoshiki basically could tango with stronger versions of Naruto and Sasuke, while Kaguya struggled with a younger, inexperienced Naruto.

Momo wins again.

Momoshiki: 2

Kaguya: 0

Hax/Arsenal:

Both Momoshiki and Kaguya have a similar arsenal with chakra absorption, space time ninjutsu, truth seeker balls etc. The only difference is that Kaguya can switch the entire terrain into a dimension, at the cost of massive amounts of chakra:

Which is kind of pointless to use on Momoshiki, because he can just create his own portal and leave whenever he wants. There's nothing stopping him from doing that.

Momoshiki also has the inherent ability to amplify chakra by turning whatever he absorbed into chakra fruits. This gives him a massive advantage in terms of longetivity and increasing his overall AP, since he could convert Gyuki's chakra into a giant TSB. Keep in mind that Kaguya needed the chakra of the entire planet to perform the same feat:

I'll give points to Kaguya though because Momoshiki can't just force the terrain into another dimension.

Momoshiki: 3

Kaguya: 1

Intelligence and overall skills:

This is the one thing that Momo and Kaguya are equally bad at.

Kaguya is literally a dumb blonde who relied on Zetsu too much and got outsmarted by Naruto a gazillion times. Momoshiki only knows how to spam chakra amplification and has zero skills aside from maybe tenketsu, which all byakugan users have inherently.

And they both have their fair share of anti feats as well.

Vitality and longevity:

Kaguya is immortal.

While Momoshiki is very much killable.

So Kaguya gets the win here.

Momoshiki: 3

Kaguya: 2

However, Kaguya can still be sealed by six paths chibaku tensei.

And guess who has a rinnengan with six paths chakra?

Momoshiki: 4

Kaguya: 2

My boy Momo wins this battle.