A very important difference is that the P's framelines are reflected, while the 7-series' are projected.
On the P, the framelines are etched in silver on a plate inside the rear eyepiece -- you can see them if you look at the camera from the back -- and then reflected back toward your eye from a semitransparent surface at the front of the viewfinder.
On the 7-series, the framelines are clear lines on a black opaque mask (it's actually a tiny piece of lithographic film onto which the lines are printed photographically!) This mask is lit from the front by the ribbed frosted window between the viewfinder and rangefinder windows, and the light that shines from it through the clear framelines is projected optically into the finder by a diagonal mirror.
While both systems work, the 7's projected-frame system produces framelines that are more evenly lit and easier to see under a wider range of viewing conditions. The P's reflected framelines can blank out under some lighting conditions, or if your eye isn't correctly aligned behind the eyepiece.
The 7 also has a longer effective base length for its rangefinder, permitting greater focusing accuracy. And it has an external breech-lock flange around its lens mount; this flange is needed if you want to use the Mirror Box 2 reflex housing or the gargantuan 50mm f/0.95 normal lens, neither of which will fit on older models without the breech-lock flange.
