As backup either will do. I use a GPS12 (the granddad of the 76) no WAAS on it, but having had a play and overlaid the "snail trails" of it & the WAAS equipped Lowrance, I would suggest that if the accuracy is that important you might be safer getting your crew to hang over the bow!
Only thing I would say about maps on small screens, having tried a GPS-V, I have found the less cluittered screen & a set of proven wayoints (see next paragraph) with the "big arrow" works better when moving.
OK, the 12 is "waypoint only" , but at 24 hr (real) battery life I can live with that. I have been building a set of waypoints to make a "join the dots get out of jail" map, theory being that betwee nthe waypoints & the snail trail (see below) I can get myself back in this random fog that descends as my battery dies.
Not so sure about the newer Etrexes, but one good thing about the 76 is that the big round connector on the back means that you can power it from the boat's battery, and when / if it dies, the Garmin will instantly switch to AA power. (Means it's recording your route all the time with no fear of battery life).
You can then use the Data link to feed your VHF - Granted that's not going to be much use with a flat battery which will take out the VHF as well, but it might help keep your Red button in a more useful state if your main plotter throws a wobbly.
I have a data patch in the console so it's a 2 min job to swap connectors - I can even if the GPS pod on the a- frame dies or the cable to it chaffes through feed the plotter from the garmin's data out, or any combo of VHF / Garmin / Plotter.