If you have the cash, the RPMs are my first choice all around wise. I've built with irons, but good prep runs close to the cost, and you still have iron heads weight wise, and in most cases, an open chamber. Magnums or Magnum R/Ts wouldnt be bad, but std Magnums may not move eough air for decent numbers above 5K rpm in most cases. So first choice, the closed chamber RPMs.