I thought it would be a 301 as then google will pass on the PR since 301 is permanent redirect.
I know when you create an alias that the fear is that, although you won't be penalised for duplicate content, but that they may only credit the alias domain and drop the higher PR with more bank linked one, since they will likely only use one domain, but not sure if this is the same with
www. domain.com or domain.com