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

Chongqing hot pot is mainly butter, supplemented by vegetable oil. It tastes mellow and likes to use old oil.

Chengdu hot pot is mainly vegetable oil, with rich aroma, mostly spicy.

Chongqing hot pot pays more attention to spicy taste. The two kinds of hot pots have their own characteristics and can be selected according to personal taste.