Depending on the cab, a 12 can kick a 15's butt or vice-versa. Question: How loud do you need to play low? If it's in your living room to practice, you have many options open. If you want to go low in a band with a live drummer, choices dwindle rapidly for single driver cabs.
*** Edit: I forgot the title after I read your question. You're trying to determine between two cabs from the same manufacturer, so scratch the comment about cab design considerations. The question remains: How loud do you need it?
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