Whether to include one card over the other comes down to specific environments, and larger Cubes could certainly facilitate both, but the point is that Callidus Assassin is a very real card for control decks that would like to spend their turns leaving up mana for counterspells and removal
The Dragonite is my new wallpaper

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Its first attack, Metal Blast, requires one energy and deals 40 damage plus an additional 40 damage for every Steel-type energy attached to it