Go for the 550! We all know that the true rear wheel horse power will be around 300 on a rear wheel Dyno. I can attest to that being my car was around 650 @ Crank and produced 415 @ the wheels.Heres a link to the Dyno shop we used for the testing, they give greater detail of the Dyno details
http://www.ktrperformance.com/dyno_t...ter_graphs.htm