mark sisson's diet is just a variant on the 'paleo' diet, a hodgepodge of different diets that revolve around the same theme of no grains, legumes, modern foods, etc. his book is called 'primal blueprint'.
which can cause some confusion with the 'primal diet', a totally different animal. the primal diet is outlined in the book 'we want to live' by aajonus vonderplanitz. the book itself is a bit simplistic compared to actually consulting with aajonus and getting an individualized program to deal with your specific problem. the primal diet is an all raw proposition that includes raw dairy, raw butter, raw eggs, raw meat, raw fish, green juices, some raw honey, and some raw nuts. it is much more formal than sisson's diet where you just dive in and start eating steaks and ribs.
i would say that sisson's diet is more for the masses whereas the primal diet is more medically oriented toward people with serious or terminal health problems.