How many DOFs (Degree Of Freedom) do you need?

DOF = Degrees Of Freedom: 3 rotations and 3 translations.

This key question is of course directly related to the amount of investment you are prepared to make. However, a dynamic platform can't be changed every year, so it's a good idea to choose a model that will cover all the areas of simulation that you'll be tempted to explore.

For aircraft/helicopters/Roller coasters/spacecraft

6DOF is absolutely necessary. The 3DOF platforms (roll, pitch and yaw) are lacking in up-down, side to side and forward and backward movement, which is essential for feeling the runway, the bumps, the air holes, and other haptic experiences.

For simracing

At least 4DOFs are required, but with 6DOFs you still have access to the best in flight/space/Roller coaster simulation and more realistic e-racing.