well, I think they both use the same technology for the sound and action. I'd say about 98%,, either way, you wouldn't know the difference, and I wouldn't let it alter my buying decision. only reason the p35 is lower end is because it doesn't come with the built in stand, pedal..So its not necessarily a low end, the internals are very similar.
if the clp-840 is in good condition, and relatively the same price.. I'd go with that.. unless you have room issues and want to store it away when guests come or move it around the house,, in that case go with the p35.