I would suggest either a MATLAB Function block or Stateflow. Simulink and if action susbsytems are OK for relatively simple logic algorithms, but for that sort of size, it will become cumbersome very quickly. Whether MATLAB Function or Stateflow is a personal preference: whether you prefer to express your algorithm graphically (and animate it during simulation for debugging purposes), or textually. Both use the same technology and so should give the same performance.
HTH,
Arnaud
