In my experience, it's always better to use server-side scripting, as you have better control over what the user will see client-side. If you're using JavaScript to accomplish something integral to content displaying correctly on the page, and the user happens to have JavaScript turned off, or has a slow computer, or a slow connexion, your whole site is lost on that user. Designers/developers themselves can be an especially difficult audience to reach when they inhabit the users' role, because they have even less patience than do 'regular' users for scripts that take a long time to process, or sites that rely too heavily on client-side scripting. Myself, if a page is having trouble running it's scripts, or if it seems the scripting isn't seamless enough, i'm gone.
Ultimately, as has been voiced by others here as well, PHP and JavaScript are suited to different applications. PHP is superb for database interfacing and content-management, whereas JavaScript is really the only alternative to Flash when it comes to user-interface interactivity...