I detest Chongqing. The hills are theoretically analogous to San Francisco, but Chongqing manages to leech all of the charm (if it ever existed) from them. And Chongqing’s version of Sichuan cuisine is hot to the point of absurdity. It is a frantic city for no good reason.
But I usually encourage people to visit both Chengdu and Chongqing for the sake of compare and contrast. The two share a similar history and have an intense rivalry between them. With 36 hours, however, go to Chengdu and enjoy its congeniality that is unusual for most Chinese cities, and perhaps take a day trip to Leshan.