I've had both the Canon P and a Nikon S3, and sold the Nikon as the RF patch was so hard to see it was a real pain to focus. I think the VF/RF just needed a good cleaning, so if you get a good copy of either camera you may not find much difference as far as useability is concerned. I kept the Canon as the VF is nice and clear and the RF patch is OK, but still not as good as a Leica M. If you like screw-mount bodies, the Canon is very smooth to operate and has a lot of nice features, especially for the price. The Nikons are a bit more expensive and a big advantage of the Canon is that you can use any LTM screw-mount lens. Nikon even made most of their rangefinder lenses in an LTM mount version. You often see the Canons with wrinkled shutter curtains, but that shouldn't be a concern as they made the curtains from a very thin metal and it shouldn't affect the operation of the shutter as long as there are no significant dents or holes.
If you need service on any Canon screw-mount camera, Youxin Ye does a great job at a very fair price. I just had him replace the curtains and clean the finder on my Canon IVsb and it's now functioning like new.