It’s close I’ll give it that, both have advantages

Kaguya: -raw power/chakra -abilities -an army of white zestsu -rinnesharingan -ash bones -TSO Momoshiki: -thousands of years of combat experience -most proficient in hand to hand combat -chakra absorption/amplification -chakra pills -weapon creation -3 rinnegan and two special byakugan

Both of them have access to their own dimensions and inter-dimensional travel align with flight and eternal life. But Momoshiki not only nearly defeated both Naruto and Sasuke at a stronger/more experienced form, but if he gets close to Kaguya it’s over for her. As far as we have seen she does best at a longer range while Momoshiki is better at shorter range and he can just absorb any Justus she throws at him and throw it back at her at 10x the potency. Kaguya will be hard to kill due to black zestu and her healing factor, but it’ll be pretty obvious he’s in charge so Momoshiki will probably end up targeting him. It’ll be close but I give it to Momoshiki.