i use an RP100. lots of cool (as well as cheesy) sounds in it. all i can really complain about it is the occasional noise. i don't like its noise gate too much.
but had i the choice back then, i would have picked the RP50 over it, since all i use are reverb, delay, chorus, noise gate and a few amp simulations. but that's just me. your mileage may vary.
as for the zoom 505... it seems like a quieter unit, probably because of the ZNR. and a friend of mine is really content with it. i also used it a bit, and seemed good for its price.
not much help, am i?
well, the bottom line from my point of view is that, assuming all three units are in good qorking condition, it'll be hard to go wrong with any of them.