You'll probably hear strong disagreement to the following (especially from the Nikon contingent!) but having owned and used both, here's my take:
-- There's no difference in sturdiness. Both are superbly constructed cameras. The P has stainless-steel shutter curtains, which can "dent" if poked while loading film, but there's no risk of burning a pinhole in them through exposure to bright sun. The S-2 has fabric curtains which can be pinholed, although it's not very likely as long as you're aware of the risk.
-- Both have 1:1 viewfinders. The P's is significantly brighter, and includes parallax-compensated bright framelines for 35mm, 50mm, and 100mm lenses. The S-2's finder is more contrasty and has a longer rangefinder base length (for theoretically more accurate focusing) but only shows a fixed, non-compensating etched frameline for the 50mm lens.
-- The P is much more convenient to use. All its shutter speeds are on a modern-type single dial, which does not rotate when the shutter fires. The dial is click-stopped so you can change shutter speeds with the camera held up to your eye. The P loads via a hinged back. Its shutter release button is in a more comfortable location (and accepts a standard cable release) and its rewind crank has a large, rotating grip on the end for faster film rewinding. It sets X flash sync automatically when you set the shutter to the X position on the dial.
The S-2 has separate fast- and slow-speed shutter dials, and the fast-speed dial rotates as the shutter fires; if you carelessly let a finger drag against it, your shutter speed will be off. You have to lift and turn the fast dial to change speeds, meaning you have to take the camera away from your eye and look at it. The S-2 loads via a removable back-and-bottom which you have to hold or stash while loading. Its shutter button is in a finger-kinking position on the back edge of the top cover, and needs an adapter to take a standard cable release. You grasp the rewind crank via a small, solid nubbin on the end, which doesn't rotate as you turn the crank, making it more difficult to keep hold of it. Flash sync uses a separate selector, which can result in blank images if you accidentally knock it to the wrong position without noticing (ask me how I found this out.)
-- The S-2 was Nikon's most popular rangefinder model, but it was nowhere near as popular as the P (almost 57,000 S-2s produced; almost 88,000 Ps produced.) Also, because US collectors still regard Nikon as a more prestigious name, prices for Nikon items tend to be higher than for similar Canon items. That means that you'll almost always pay less for a Canon body or lens than for a comparable Nikon body or lens in similar condition; in other words, you get more for your money buying Canon.
Does that mean you shouldn't buy an S-2? NO!!! It's a very worthwhile camera to have, and it makes an interesting comparison to the P because it shows how two successful Japanese manufacturers applied different solutions to the same set of technological problems during roughly the same time period.
Just don't buy one thinking it's "better" than the P. Buy one because it's a great camera that's different from the P, which is also a great camera.
