I've (finally) tried both FreeNAS and NAS4free and I'd like to share some thoughts and experiences. Both of these are "NAS-in-a-box" products intended to be installed on computers with a large number of drives, which they will export to the world in a variety of protocols. Both are based on FreeBSD, both fully support ZFS and they even share a common history.
The tl;dr of this post would be something like "yes, they are similar enough to be interchangable, BUT...", as I actually cannot give a more specific advice. Read on for why.