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I would actually argue that Armada Wurm is the more proactive of the two and that Centaur Healer is more of a sideboard card (though it depends substantially on the tenor of the metagame)

Legality depends on the tournament format -- Fear of Going 0-2 Drop can be legal in some formats and banned or restricted in others

Now youre drawing a card every turn while also putting your opponent on a clock

In fact, Primeval Titan has been the face behind some of the most degenerate decks in Modern, starting with the Cloudpost deck that was prevalent in the initial days of the format, to the Red-Green Valakut deck, and then the Amulet Titan deck both in its degenerate form with Summer Bloom, and the more toned down version we see today with Dryad of the Ilysian Grove and Urzas Saga

Thanks to starting with 40 life in Commander, even Vampiric Tutors additional cost of losing two life is more than an acceptable sacrifice for the ability to tutor any card to the top of your library