Having used both, I'd say it a bit like being bitten by the cat or the dog. Little difference.
All in all I's say Navionics is a bit more versatile in using it at other places (iPad, Computer) but both their PC-planning software leaves very much to be desired.
In the chartplotter the differences are minimal. Just very expensive to buy and to update.
I don't really like the original C-Map cartridges. they are still used but quite outdated these days.