In all the tests I have seen, each has their strong points, but C-Map usually gets the nod overall. I have c-Map in my chartplotter, and they are good - great detail and plenty of harbour information etc..
I am a great fan of C-Map, looks and feels more like a chart to me. Best to get a dealer to show you two plotters with each installed for your usual sea area and go with what you feel most comfortable with
with Navionics you get more geographical area per pound...
.... with C-Map you get more detail and better icons/graphics and info.
Navionics have a new system called Gold Card soon to be released. ... also some new Geonav plotters.
C-Map will shortly have a newer and easier way of getting charts. They also have PC Planner allowing you to use you cartridges on the PC and to transfer data between PC nad plotter.