it vary your needs. If you would like to go more deep into NIS and NFS in details, i prefer/recommend you go for SunOS/Solaris.
This site can help you better to diffrenciate and have some exact picture on which OS you should go with:
http://www.unixtools.com/unixhelp.html
By the way Intel Version of Sun is diffrent from Sparc version. You can get more info from here:
http://www.sun.com/learnabout/solaris/