I'd like to host a website on Amazon S3. What's the best practice to make a site accessible from both
You should choose one version to use and 301 redirect from one to the other. I don't know if you can use a htaccess file on S3 but if not you may be able to do the redirect from you domain name's control panel.
Amazon offers a free micro server for one year. http://aws.amazon.com/free/ you may be better off setting up a public AMI LAMP stack and have the freedom to configure your website however you'd like.