Create subdomains best practice?

I have a client that will be in need of many subdomains with Wordpress installs and e-mail accounts. Is it best to create the subdomains as actual subdomains (ex: subsite1.mainsite.com) to the root domain (mainsite.com), or create each subdomain as a separate hosting account? Also, if creating them as actual subdomains, is it then better for security or other reasons to have the subdomains created outside or inside the root / public_html folder?