Depends on what you’re preloading. If you’re preloading UI images (in PlayerGui), use it on the client. Else, if you’re loading them in Workspace so that everyone can see, preload on Server.
It depends on what kind of assets server necessarily needs to load. The server only needs to preload once. I hardly think there is anything to preload, because client usually takes care of the visuals which are most likely preload-intensive for gameplay-related aspects.