Nexenta is based on Solaris, which is the home-base of ZFS. FreeNas and ZFSGuru are based on BSD.
http://www.zfsbuild.com/2013/01/25/zfsbuild2012-nexenta-vs-freenas-vs-zfsguru/
It is amazing that FreeNas 8.3 can keep up with Nexenta now. ZFSGuru can´t keep up with the other two and according to the benchmarks the difference is quite big. The makers of ZFSGuru are looking now if there is a benchmark error or it is because of the version (0.2 Beta 7) used in the benchmarking. Since ZFSGuru comes with a full version of FreeBSD, there are a lot of possibilities of tweaking with the commandline.
However, I personally prefer FreeNas on a production system since ZFSGuru is still more in development (but stable according to many users), missing alot of features in the GUI, and they (still) are missing a good manual (like FreeNas 8) which makes life for noobs like me easier.
Hoenjet 