A good way to think about it is to decide which will motivate you to write. Will ticking through 12K every month get you to the page regularly, or do you need the set number of days to get yourself to write? Also consider if you'll "phone it in" on the Habit Pledge, writing a very little bit and calling it done.

There's a lot to consider about which goal will motivate you to create a habit, and choosing it is entirely subjective.

That said, based off your current word count, I think you'd have no trouble nailing 150K if you think a word count would be more motivating.