I just replicate my whole ePO Server and SQL Server VM's, en toto.
Key component is the database, and mirroring is the easy way to do that.
Clustering across datacentres, depending on the connectivity, might be more pain than it is worth.
Really depends what are you objectives. Have you considered building an AH in the other location ? It could provide some service if the ePo server is down and be repointed to the mirror if the site is down.