I have both. When I bought the lakemaster 2 years ago, there were only like 3 lakes in Missouri that were "HD" lakes. If I remember right, clearwater, pomme, and truman? The HD maps on Lakemaster were more detailed than the navionics maps. However for the non HD lakes, the Navionics were far more detailed when it came to contours and offshore structure. For me personally my choice for bull shoals and table rock is definitely the navionics. As far as roadbeds and building foundations, truth be told, my favorite source is still the paper hot spot maps. I look for them on the paper map, and then pin point them with the gps and side imaging. What I did is bought one of each as I have two graphs on my boat. I just swap the two map chips between the two graphs depending on what lake i'm on. If i'm fishing pomme I'll put the lake master in the console graph, bull shoals, vis versa. Hope this helps. I know there have been long drawn out fights/arguments on the BBC Humminbird forum over this topic.
Oh and another advantage of the Navionics chip, is you can download the Navionics PC app for free after purchase. It allows you to browse all of the maps from your desktop computer. Its really nice. Lakemaster also offers a similar computer program but you pay like 60 bucks for it.