The Sitex unit reads/plots both C-Map & navionics, the Furuno only C-Map.
I think if it were me and the unit was strictly for redundant backup, I would go with a plotter that used whichever charts that your main units did not. ie: if your main units are using navionics or c-map charts, I would have a redundant unit that will accept SD cards and can read the official NOAA raster & vector charts. (laptop, Ipad, etc) It seems pointless to me to have redundancy in electronics without redundancy in the charts you can use.
Agree 100%...I believe the SiTex unit will meet those criteria and as Bill mentioned, it also has the larger screen. You just don't run into that many recreational boaters using SiTex for whatever reason although I've enjoyed their chartplotters on past and present boats.
I also do not believe there is as much a price tag associated with the SiTex name as there is with Furuno even though they appear to be very similar in function and capability.