Would be best to get enough equipment stats to not need either. But guess it depends on how fast the haunts get completed.
Pots since the cost of a single mith spell can be equivalent or more (depending on how many items you get in) than you make in a single Haunt.
However, I personally have equipment to help my success rate when hitting Haunts. Just need that damn bow to help me when hitting the higher tier EBs with a better success rate.
The price for mith with guild hansels can be pretty cheap. Around 500 mil per mith and the cheap spells last 5 hours. If you're using a lot of xtals mith would be cheaper then pots.
Depends. For example, with my size, one mith would cost me just under 500m, and would get me a 25m attack bonus, same as a locust swarm. So, 500m divided by the 8.5m for a swarm means, if I attack 59 times, then they are the same price. Less than that, and I should use pots. More than that, and the mith is cheaper. If you have more than six mith, or your price for one mith is significantly higher, your mileage may vary.
You simply have to work your way up. Start with the smaller pieces and slowly go for the bigger ones.