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You mean having both serve the same content? Yeah, the downside is SEO. Google picks up both versions and labels it duplicate content.

You can put a canonical meta tag on each page, but as I understand you lose a few percent of the page's value. 301 redirect is the only way to conserve all of the value.

Not an expert, just repeating stuff I've been told by our SEO guys.

According to Matt Cutts, who works with the Google Search quality team, you actually lose some of the link value passed through a 301 redirect.

There's no penalty for redirecting an entire root-level domain to a subdomain, as long as your sitemap and internal linking strategy all point to the subdomain.

From Google's perspective, the problem with 301s is that they take longer to load (since you've got to request two separate URLs) and they make your URL structure less stable. Neither of those is a problem if you make it clear to Google that your content belongs on a subdomain.

You'd still have to choose your preference, either to redirect to non-www or www. I prefer www, but that's just me

No as long as you 301 to one of them.