The difference between the Edelbrock Performer RPM and the Performer manifold on a 350 CI Pontiac engine will not make any significant difference in overall performance. What little difference in performance that you will notice with a 350 CI Pontiac engine above 3,000 RPM is not worth the effort and cost it will take to change manifolds.
A Performer manifold is a replacement for a stock manifold. The Performer RPM manifold is more suited for a 400 CI Pontiac engine that has a better bore/stroke ratio and can take advantage of the short runners and larger plenum area at high RPM, as the name suggests. However, it will not operate as well at low RPM and in normal driving.