As I read it, RDM is the recommended one (across boxes) if you want to use a "VMware" type of shared storage. But it also states:

"Use of software iSCSI initiators within guest operating systems configured with MSCS, in any configuration supported by Microsoft, is transparent to ESXi hosts and there is no need for explicit support statements from VMware"

So both are viable. iSCSI within the guest is probably easier to manage. Go the RDM route if you have some SAN functionallity or tools like SAN based snapshots/replication that you want to use.