I would love to have synthetic vision. I imagine it is a great safety feature but it is currently too expensive
3D synthetic vision (uncertified) is available as part of the PocketFMS app currently in the Android Market (erm..Google Play! now!) and shortly the Apple app store.
I gave up drawing lines on maps years ago for flying in Ireland. If I go further afield, I will draw a line on the chart.
In all cases I will have a plog, even the most simple of flights, because my GPS software (PocketFMS) will produce that plog for me with no effort, along with a list of all the freqnecies that I need.
I always carry a paper chart though (though giving consideration to only carrying a digital chart in the future, with a second digital backup). So if for some reason GPS is unavailable, I still have my plog, and paper chart.
If you choose the right GPS/flight planning system combination, then producing your plog and 'programming' the GPS are just one task one two. So having your plog is no more effort than just using GPS.
As for which one is primary, and which is secondary is a mute point. Know where you are according to both. If one is suggesting that you are somewhere different than the other is suggesting, then figure out which is right and which is wrong!
Most likely the GPS is right, but they can lose signal, freeze up or have the incorrect route put in! However your GPS is unlikely to continue functioning, updating your position, heading and speed, and show you in the wrong position! Most unlikely indeed!
Most errors when using GPS seem to be in vertical navigation.