It depends, I have used both. Remember that presenting storage via NFS is different from presenting iSCSI. With iSCSI, the VMware hosts see block devices which will be formatted with the VMFS (Virtual Machine File System). NFS presents a file system to be used for storage.
A lot of your choice depends on the hardware/software you are running. For example, if you use the NFS server role on Windows Server to present storage - it's going to be a bad experience. Microsoft's implementation of NFS is not very good. If you use a Synology device and present iSCSI to vSphere, you'll hit severe performance issues! The above are just known issues with those vendors. Generally if you buy a dedicated server (Dell, HP, Supermicro) or build your own and use quality network cards (Intel, etc.) you will see similar performance regardless of which protocol you use.
One thing to mention between iSCSI and NFS is, with iSCSI you can utilize multipathing and load balancing to provide redundancy and reliability; I believe vSphere still connects via NFS v3 which means you won't have those options.
I personally prefer iSCSI, I would rather let vSphere manage the file system