First off, look at the Canon Museum website for an overview of Canon rangefinders:

The biggest differences between the IVSb and the P is in the film advance and viewfinder. The P has a "modern" lever advance while the IVSb has the a knob advance. The viewfinder of the IVSb is often referred to as a "squinty type" while the P is much larger and has reflected framelines. Do take a look through both viewfinders before making your decision. Any old camera can look cool but how a camera fits in your hands and how it operates is probably more important.

Jim B.