Redirect or Mirror Domain?

I have a new site hosted on DreamHost with the domain registered elsewhere. The new domain is a .org domain. The nonprofit I built the site for has had a site hosted on Yahoo for several years with a .com domain. We have decided to quit Yahoo hosting and have the .com domain point to the new site with the .org domain. I plan to add it to our hosting account as a subdomain. Since most of our marketing mentions the .com site and visitors have been going to the .com site and are familiar with it, would I be better off having the domain redirect to the new site or mirror the new site? We plan to change the DNS to point to Dreamhost’s name servers soon. Thanks for your response.

Pick one of the domains to be your “real” domain name, and redirect the other domain to the real one. Mirroring isn’t what you want for this situation.