The framelines of the P's viewfinder are reflected, not projected like on the M-series Leica and the Canon 7/7s. Because of this, some viewfinders of the P have aged well, some haven't. Always look at through the viewfinder of a Canon P before buying.
That being said, the P has almost a cult-like following. A simple but very high-quality 35mm rangefinder camera. The Canon 50/1.4 is a superb lens too. It's relegated my collapsible 50/2.0 Summicron to my closet.
But try before you buy, (similarly some SLRs have damaged penta prism optics).
CRR at Luton will refurb a Leica M finder to be better than new, Ive looked through one, but expensive unless you get the camera for free.
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)
The Canon P is closer to the Nikon S3 or SP or F2.
It has a metal shutter like a late SP, but you are unlikely to burn a fabric shutter unless you are careless, hands up...
A non rotating speed dial like a S3, more critical limitation unless you have good finger control.
Fast loading like a Nikon F2 (SLR), some people need this...
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.
You need to pay for either....
The Nikon S3 and SP models were pro cameras and competed with M3 and M2, Leica were not able to make a popular SLR system to compete the Nikon F and follow on F2,... Leica were also not able to compete with Canon SLRs. Zeiss did not do so well either.
Leica did not go to production assembly until the M4-2... they were lucky to survive.
Most of the viewfinders of of 35mm range-finder cameras that era were crap by modern standards.
If their respective conditions are reasonable, they are both good and very usable cameras. The P has a decent finder with more frame lines, but in my view the finder is not quite as "ergonomic" as on the S2. The S2 has only 50mm framing (requiring external finders with non-50mm lenses), but the S2 finder has more eye relief and is just very eye friendly. If one is changing lenses frequently, the P might be a better choice. The P is also a less expensive camera and probably a little newer than the S2. Nikon rangefinder mount lenses are fairly plentiful, but not quite as easy to find in all focal lengths as LTM lenses, although some of the Nikon fit lenses are a compelling reason in themselves to support a Nilkon RF mount body purchase!
The Canon P and Nikon S2 are not similar cameras. The S2 is similar to the Canon L series, the Canon P is similar to the Nikon S3, the P and S3 both have similar frame lines in the viewfinder.
I have repaired numerous examples of both cameras, and both are similar in quality. The advantage of the Nikon is that it can be wound, focused, and shot with one hand. The advantage od the P is that LTM lenses are more plentiful. That said, the Nikkor lenses are better made than Canon lenses. The lubricant in Canon lenses attacks the coating inside the lenses, first causing haze, and eventually ruining the coating.
The S3 (if you can find one for a good price) is much better than the Canon P. The mechanics are more refined, and the S3 is a smooth operator. I have a couple of S3 cameras, and they are actually smoother and quieter than my Leica M3 or M4 cameras.