Multiple domains for one site: alias or redirect?

Question

I'm setting up a number sites right now and many of them have multiple domains. The question is: do I alias the domain (with ServerAlias) or do I Redirect the request?

Obviously ServerAlias is better/easier from a readability or scripting perspective. I have heard however that Google likes it better if everything redirects to one domain. Is this true? If so, what redirect code should be used?

Common vhost examples will have:

ServerName example.net
ServerAlias www.example.net

Is this wrong and should the www also be a redirect in addition to example2.net and www.example2.net? Or is Google smart enough to that all these sites (or at least the www) are the same site?

UPDATE: Part of the reasoning for wanting aliases is that they are much faster. A redirect for a dialup user just because they did (or didn't) use the www adds significantly to initial page load.

UPDATE and ANSWER: Thanks Paul for finding the Google link which instructs us to "help your fellow webmasters by not perpetuating the myth of duplicate content penalties". Note, however, this only applies to content ON THE SAME SITE, exemplified in the article with "www.example.com/skates.asp?color=black&brand=riedell or www.example.com/skates.asp?brand=riedell&color=black". In fact, the article explicitly says "Don't create multiple pages, subdomains, or domains with substantially duplicate content."

Answer 1

Redirecting is better, then there is always one, canonical domain for your content. I hear Google penalises multiple domains hosting the same content, but I can't find a source for that at the moment (edit, here's one article, but from 2005, which is ancient history in Internet years!) (not correct, see edit below)

Here's some mod-rewrite rules to redirect to a canonical domain:

RewriteCond %{HTTP_HOST}   !^www\.foobar\.com [NC]
RewriteCond %{HTTP_HOST}   !^$
RewriteRule ^/(.*)         http://www.foobar.com/$1 [L,R=permanent]

That checks that the host isn't the canonical domain (www.foobar.com) and checks that a domain has actually been specified, before deciding to redirect the request to the canonical domain.

Further Edit: Here's an article straight from the horses mouth - seems it's not as big an issue as you might think. Please read this article CAREFULLY as it distinguishes between duplicate content on the same site (as in "www.example.com/skates.asp?color=black&brand=riedell and www.example.com/skates.asp?brand=riedell&color=black") and specifically says "Don't create multiple pages, subdomains, or domains with substantially duplicate content."