I believe both the NOI (Black Islam) and more mainstream Islam are active in prisons.
NOI is actually a sect of Islam that is separate from Sunni and Shia Islam. In addition to believing in Allah, Mohammed (hell be upon him), jihad, not eating pork, and Jew-hatred, black Islam also teaches that blacks are the master race, that the black race is descended from the Arabs via the tribe of Shabazz, that Wallace Fard (ys"vz) was an incarnation of Allah (very heretical to the main branches of Islam), that Elijah Muhammad was a great prophet, that whites were invented 5,000 years ago by an evil black scientist, and that a giant spaceship called the Mother Plane, that is controlled by Allah, will destroy them all. These additions are considered quite crazy, and even blasphemous, by most other Muslims.
There is little difference between the two in terms of their general hatred and danger to mankind on a personal level, but black Islam is considerably less sophisticated, and much "loonier", than regular Islam from an organizational standpoint.