Before the release of Neon Dynasty, this was a pretty easy question to answer with Blood Moon being much harder to answer than Magus. However, now that Boseiju, Who Endures has not only entered the format but sees play in the main deck of both Tron and Titan, the right decision becomes more complicated. Magus of the Moon dodging not only Boseiju but other Disenchant effects like Force of Vigor has suddenly become incredibly relevant and I find myself spending more time with the decision. That being said, when selecting one or the other, this is the most important question to answer:
Am I playing my Moon effect in the main or sideboard?
For the most part, if you’re wanting to play a Moon effect in your main deck, you’ll usually want to play Blood Moon over Magus. While you lose percentage points against the big mana decks with this decision, the reality is that preboard Modern games are very much creatures and creature-based removal at the moment, and Magus usually ends up being an incredibly fragile threat game one. The exception to this rule is if your deck gains advantage from having more creatures in play. This would be the case for a deck with Equipment like Boros Stoneblade or a brew with Collected Company or Unearth.
If you’re playing a deck that wants to play Blood Moon in your sideboard, then the question becomes: do I want to play this card to hedge against any matchup besides Tron and Titan?
I’ve found recently that since the Omnath decks are shockingly resilient to Blood Moon, there are few matchups you want the effect for besides the big mana decks. If this is the case, then it’s an easy choice to play Magus. If there are other matchups in the meta that you want a Moon effect for, then you need to evaluate how important the effect is for winning the matchup and how hard it is for them to remove Magus versus Blood Moon. As a general rule of thumb, decks with red removal and Fury or Solitude can easily answer a Magus and you’d usually want to play Blood Moon instead.