I need to add some storage to backup some KVM VMs.

The storage I want to use is on my FreeNAS box.

Should I just make an NFS share? Or is there any benefit to do this with iSCSI instead?

All of the backups run at night. At present, they all back up to Amazon S3, but I want to add another backup.

Typically ISCSI will give better performance.

NFS should be fine.  With iSCSI you need to shut down or manually disconnect the client before restarting the NAS.

iSCSI will typically be faster in an ideal, optimized environment.

However, there are other factors to consider:

 - Do you have enough CPU to maximize throughput? (Probably, if it is a server.)
 - Can your storage support that level of I/O? (Probably, if it is modern drives on a good controller.
 - Can your network support that level of throughput? (Probably maybe. With good hardware you can saturate a 1GB link.)
 - Do you want block centric vs file centric storage? (This is probably the most important. Lot of pros and cons.)

As with most things, it will only be as fast as the slowest bottleneck.

I vote nfs so you don't have to mess with resizing the volume.