Hi there.

Finder of the Canon P is commonly brighter, less dimm than the Nikon S2 finders and the RF patch better to see.
Plus you got the 35mm framelines.
The Canon P is heavier and seems to be more sturdy than the S2.
M39 lenses are more common and less expensive in same quality than Nikon S mount.
A working Canon P with an attached meter isn't to beat as classic user RF camera. You want a TTL meter, take the modern follower, Bessa R (but take the black ones, they keep better when the color comes off in heavy use)