I think this is a personal question and really depends on how you feel when you do one vs. the other. I tend to go with 14 BUT I am pretty tall and my legs are really long, so 12 doesn't do much for me.
Also, I agree that if you already have the club step, you can swap out the risers, so you only need to buy the smaller one (or just the topper, depending on how many risers you already have).
Marie