301 or alias?

Hi guys,
I have a client who has multiple domains all pointing to the one website. At the moment they have all these additional domains (5 of them) as domain aliases. My job is to SEO the site and drive more traffic to it.
I havnt dealt with domain aliases before, but from an SEO angle, what is the best way to handle this? Should I leave them aliased, or use 301 redirects instead?

If a domain is pointing to another domain it will always have that PR. If I point a site to google, my site will have a pr of 10. All you need to do is check that Google is being redirected.