If you're playing very aggressively, esp. with a hero-based strategy, and still have a decent hand size (4 or 5), Scavenger can be preferable turns 2 and even 3 (just make sure all your stuff is on the board, not hastily dying and cluttering up your next cycle, or make sure you can draw a lot of cards!).
But the rule is that whichever one you chose will be the one you regret, as you'll either draw the card you need and it's too expensive, or you'll have the money but not the cards you needed. (;