What's not mentioned here is that the VMFS filesystem and NOT iSCSI per se is what makes storage share-capable. Not all filesystems allow access from more than one system at a time. Clearly NFS does. Something critically important to note is that iSCSI is block-level over IP, while NFS is a file-level protocol/filesystem. There is a ton of advantages to NFS, that are simply not an option with iSCSI. I work for Nexenta, and every day we spend hours in debates of what is better. Ultimately, it does come down to what your environment needs, and the level of knowledge of iSCSI and NFS by the staff expected to support it.
Multiple iSCSI initiators can connect to the same target, and assuming that configuration allows multiple clients can access the same LUN. VMFS allows for this, and that's how DRS clusters work. Without this ability you could not do a lot of what clustering offers, such as Vmotion, etc.
NFS is by default a shared filesystem. When you build a datastore on NFS, assuming you export NFS to all notes in your DRS cluster, all files stored on the NFS datastore are accessible from all hosts in the cluster.
Again, difference is NFS is a filesystem. iSCSI is a SCSI-3 protocol delievered via IP. Not all filesystems delievered via iSCSI are natively capable of being shared. NFS is natively a shared filesystem.