It mainly depends on what you are trying to achieve and whether you consider that time worthwhile. They are both great options and FreeNAS already gives you a

It mainly depends on what you are trying to achieve and whether you consider that time worthwhile. They are both great options and FreeNAS already gives you a lot of flexibility. It's a bit like with a router. Do you use something along the lines OpenWRT and mainly log in to the command line, not because you have to, but because you can then you might want to go with FreeBSD and ZFS. However, if you don't know if it is flexible enough and fits: Go with FreeNAS and fall back to FreeBSD. I think that's the best option for most people.