With rare exceptions we recommend artists spend no more than 30% of their song promo budget on YouTube so that the remaining 70%+ can be allocated towards Spotify.
Spotify is easier to trigger the algorithm and yield long-term results, which is why we recommend artists invest the majority of their promo budget on the Spotify platform.
However, it's still important to promote on YouTube, as reports indicate YouTube to be the #1 place that people search for music in the world. Additionally, YouTube can be a great place to have social proof with metrics that indicate obvious resonance with your music. This can be leveraged not only for fan discovery, but also bookings, collaborations, and many more opportunities.
If your goal is split between growing on both Spotify and growing on YouTube in order to establish further social proof, we would recommend splitting up your budget between the two.
If your goal is to yield the most listeners long term at the expense of establishing social proof on YouTube, the world's #1 music discovery platform, we would recommend allocating your entire promo budget to Spotify.