Not sure about the Navionics, sorry, but I just purchased the BlueChart g2 Vision card (Garmin). It hasn't arrived yet, but I bought it primarily for the Auto Guidance function.
Without the Vision card when you route to a waypoint it may take you over land to get there (shortest path). With the AG, it is "supposed" to avoid wrecks, shallow water, etc.
Can you do a search and see if Navionics has the same function? In my opinion, if it does - that alone is worth the price of the upgraded map/software.