Both are great products, and each have their own strengths and weaknesses, mostly depending on map availability for the the lakes or areas on a lake that you fish. I would compare data from both cards for the areas you fish, and if you cant run both cards, I would go with the one that offers the most and best coverage for you. I am running the Humminbird Solix units on my current boat and use the Navionics Platinum South card and it works great for me and those units allow me to share 1 card between the two. Check out the latest data from Navionics at the Webapp and be sure to toggle the button in the bottom left corner to turn on the Sonar Chart layer.
Navionics Webapp