We will just have to disagree.
Many Garmin units do not have navigation aids and buoys on the base map and their base maps are very poor detail. I Have not found a Lowrance unit that does not have navigation aids and buoys and I have checked a lot of units.
Aftermarket charts for Garmin are not capatibilty with other brands but Navionics charts are compatable with many brands, Lowrance, Eagle, Furuno, Humingbird Raymarine, Northstar and others.
http://www.navionics.com/GoldPlusCompatiblePlotters.asp
For the coast of California where I fish Nauticpath and Navionics are just as good as Garmins Blue charts and they are cheaper.
Nauticpath charts are just like Marine paper charts but they have points where it will tell you the tide Level and weather is a EBB or Flood tide. It also has many points where you the check the current flow and direction. They do not have satellite views or 3D views. They do have depths, conture lines, Wreaks, all the navigation aids, points of interest and much more.
They also have many local names that are not on the Paper charts from NOAA or Garmin charts on the top of the line Garmin slow touch screen models that hey had at this years boat show. An example is Elephant rock is displayed on Garmin as a rock but no name. The same with all the beaches and coves. Lowrance does show the rock with the name. Even after fishing the area since 1981 and learning 70 percent of the names of popular fishing area, I down loaded lowrance's Emulators for the M68 C a very cheap unit. Looking at the base map I found the location of many places I had heard people talk about on the radio but could never figure out where they were. So for the last 2 years when I here someone mention a place I know where it is.
Navionics Gold charts also show the entire USA Coastline. They draw the same data as Garmin but Lowrance is faster. With Lowrance you can turn on or off any data you want or do not want also.
Lowrance HD units have all the maps installed on a Hard Drive, Coastal charts inland roads and lakes with way more detail than I want.
We do agree you should go to a place that has both and compare.