I think the difference between Chongqing hot pot and Chengdu hot pot is as follows:

Chongqing hot pot is based on butter, vegetable oil as a supplement, mellow flavor, like to use old oil.

Chengdu hot pot is mainly vegetable oil, strong aroma, more only spicy.

Chongqing hot pot is more focused on spicy flavor. Both kinds of hot pot have their own characteristics, you can choose according to personal taste.