Just out of curiosity, why are you planting them in pots and not the ground?
Butterfly
weed forms a large taproot, so if you do plant them in the ground, make sure it's where they will stay...they are very difficult to transplant without breaking the taproot.
Should you decide on pots, I would use the moisture control soil. Even though they are drought tolerant once established, pots dry out much quicker and the moisture control soil will help prevent underwatering.