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Brian As far as which to remove, that's just up to him to decide. Unless I saw how the site was put together with plug-ins etc he should be able to determine which one is best to use.  Its whichever is easier for him really.  From an SEO standpoint you just need to choose one, it doesn't matter which. Hope that clarifies! -Dan

Beyond Daniel's suggestion, I'd support that with a press release and social campaign to promote the change.  This will provide immediate / short-term off-site signals promoting the new domain right away. From there, long term, I'd work on link reclamation - seeing what existing inbound links you can get changed to point to the new domain - those existing links will have more value when you do that, since it's pretty much agreed that 301 redirects leak a bit of their value. And of course, establish a long term plan for new link building to the new domain as well.