[Turbowned]

Generally speaking, a supercharger should be more reliable and less fuss because it is internally lubricated and does not have to tap into the engine oil supply. Also, boost tends to be more consistent and thus safer because it doesn't rely on a wastegate to meter boost pressure (again, generally speaking). In the FR-S/BRZ, the primary catalytic converter is located in the exhaust manifold, so that has to be eliminated if you install a turbo. Therefore, your likelihood of passing emissions without doing any funny stuff to the ECU becomes a challenge as well.

You can upgrade to a smaller pulley to run more boost, and if you max out the Vortech or Rotrex blower, like you could max out a turbo, you can always upgrade to a larger unit.