I know more of PI, as I'm going there this August, so let me add my grain of sand: Both PI and Part III are 1-year programs, but PI guarantees funding. If you want to go to PI, I've heard by a PI Professors that it's usually recommended that you contact a Prof. at PI either before going/right after arriving to begin working ASAP in your research project/essay - the last 3 months that they give you at PI for the essay is not usually enough for a good research project. Also, the PSI program is probably the most common (and "easiest"? in some sense) way to get into a Ph.D. program at PI. As far as weather goes.... well, I guess neither Waterloo nor Cambridge are known for having good weather.