Wanted: Garmin GPSMAP 557xs or EchoMAP 50s/50dv wanted

[MadTubes]

Hi there - on the search for a Garmin 557xs if anyone has one to sell or seen them at a good price anywhere? Have also looked at the Garmin EchoMap 50s and 50dv and still a little unclear as to the differences between the models - most seem to advise going for the GPSMAP series, is it worth the extra £100 or is anyone able to shed any light?
Thanks.