When it's detected that a mobile device is accessing the site it has the ability to redirect from www.example.com to m.example.com. Does it make more sense to employ a 301 or 302 redirect here? Google says a 301 but does not explain why (although usually I stick to "when in doubt, 301") . It seems like a 302 would prevent passing link juice to the mobile site and having mobile-optimized results also showing up in Google's index. What is the preference here?

Example: sitemap.example.com/sitemap.xml for pages on www.example.com.