Hi,
RCD510 has SD card slot, touch screen, 6cd changer built in (with MP3/WMA compatibility) and some versions have DAB. (Ebay £200 - £320)
RNS510 has SD card slot, touch screen, Sat-Nav, Built-in hard drive and there are a few kicking about with DAB. These are worth around £200 more than the equivalent age RCD510.
Just my two cents!