I've got the 76CSX as an AA battery powered back up for our fixed Garmin chartplotter (of which the screen is currently u/s!). It's a very cleaver and adaptable bit of kit, if a bit heavy on the batteries that it uses. It also uses a micro SD card which (with the right Garmin software) is compatible with current Garmin chartplotters but is a long way away from the Garmin compatible only chip for the fixed (GPS 2106) one.
I think you'll find that the 'S' gives you a compass and barometer (and hence altitude) reading. I take mine skiing where altitude comes into play. Not so sure of its use on a boat where the altitude reading tends to be zero (unless you're out in very rough weather!)
I've heard that the barometer side can, however, help get you through coding (where an on board barometer is a required).
Can't assist any further but hope that helps.