Each creature card in you graveyard has scavenge. Their scavenge cost is equal to its mana cost.
My question is simple: do I get to choose which side I can scavenge?
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The answer is simple: No. You must choose the card's front face (the face which has the sun symbol in the top-left corner.) In the case of flip cards, you must choose the card's normal characteristics.
711.2a In every zone other than the battlefield, and also on the battlefield with its front face up, a double-faced card has only the characteristics of the front face.
709.2. In every zone other than the battlefield, and also on the battlefield before the permanent flips, a flip card has only the normal characteristics of the card. Once a permanent is flipped, its normal name, text box, type line, power, and toughness don't apply and the alternative versions of those characteristics apply instead.