As said, it's all about timing. If I have one card in hand, I'd rather it be the Bargain, because then I can pay 7 life, fully replenish my hand, and keep going right away. With Potence, I need to wait to get those cards, giving my opponents time to anticipate it and act accordingly. Try them both out for a while, you'll realize quickly enough why the Bargain is good enough to be broken, whereas Potence, while definitely a powerhouse, doesn't quite get there.
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My Commander decks:
Chandra, Torch of Defiance - Oops! All Chandras. Prime Speaker Zegana - Draw for Power. Pir & Toothy - Counterpalooza. Arcades, the Strategist - Another Brick in the Wall. Zacama, Primal Calamity - Calamity of Double Mana. Edgar Markov - Vampires Don't Die. Child of Alara - Dreamcrusher.
Misc. EDH Stuff: Commander Cube | Zombies (Horde)
Resources:Commander Rulings FAQ | Commander Deckbuilding Guide
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