Both of these cameras would be old enough that the individual copies you may be looking at are going to determine which has a better viewfinder. I like the simplicity of the P and its "modern" features like a single shutter speed dial.
The M-3 was a major leap forward in modern camera features. It basically convinced the Japanese makers, Nikon more than Canon at first, to build SLRs to compete. The M series Leicas had enough patent protection that other rangefinder manufacturers just couldn't compete.