Lets say that one day you wake up in the Marvel Universe. As a mutant. You're not a world class power, but you're not a weakling either. Currently the world is in an uproar, hatred of mutants is at an all time high, and the US government is about to vote on the Mutant Registration Act. If it passes, many other countries will likely follow suite.
Which side would you choose, Professor Xavier and his "Mutants and humans can all live together peacefully" ideals, or Magneto and his "Humans will always hate and fear us, and it is just a matter of time before they try and destroy us, so we must act first"? Do you join the X-men, or the Brotherhood?