Your question should be "what are the advantages of M Series boards over the Eseries boards????"
Technically comparing, the M series boards are a higher version( and cheaper ) than E Series, have many additional features that E series boards do not have.
read this link for more info
http://zone.ni.com/devzone/conceptd.nsf/webmain/9C71859EB70D6E2586256ED7007CCF7F
Hope this helps
Regards
Dev