Which candidate will be tougher on China as President? Trump in the short term, and Harris in the long term.
Why? Trade, Allies, and Taiwan.
Trump will initially push more economic decoupling with China, through tariffs and actions against Chinese companies. But Trump has long been suspicious of U.S. troop presence globally, and of allies, like Japan and Taiwan, not paying enough for their support.
A Harris presidency would likely coordinate more with Japan, South Korea, and Taiwan -- moves that Beijing would find threatening.
Trump is more of a wild card when it comes to Taiwan. Many of his advisors strongly support Taiwan, but he himself could possibly let China seize it and prevent the U.S. from going to war with China over Taiwan. Harris, for better or worse, wouldn't be able to afford to seem that weak.
We don't know much about the personality of top Chinese leaders, but it's safe to assume that they don't deal as well with female leaders (the entire Politburo, the elite body that rules China, consists of 24 members, all men.)
Also, most politicians relatively inexperienced in foreign policy -- and I include Harris in this camp -- come into office thinking they can build a rapport with Chinese leaders. They mostly leave disappointed. In other words, Harris could start a dove and leave a hawk.