Good question, and I’d be interested to know if anyone is indeed earning money from PPL. I’m not, although admittedly I only have a few songs on their register from a former career 15 years ago, and they’re not big sellers. (Or indeed sellers at all, for that matter. 🙂 )
I can give you an answer about the difference between PPL and PRS, I think. PPL collects royalties on behalf of performers, whereas PRS collects on behalf of writers and publishers.
I guess that means PPL is collecting the mechanical royalty (based on the recording) and PRS the performance royalty (based on the composition), although I’m not 100% sure. It’s a flippin complicated situation, anyway!