Jim, I'm not sure how this relates, but I toured about 1,000 miles this week using the Garmin on the dash and the Tahoe's in dash mapper (I think it is a Panasonic). So far I much prefer the Garmin. The big negative for the Panasonic is that it won't give turn by turn directions on the smaller roads, even though it shows the roads and their name on the map.
A couple of miles from my house, it tells me that it will not be able to give any further directions and that I should use the compass and head in the direction of the waypoint (which is also shown on the map!). I don't know why this is set up this way. Of course the Garmin just keeps routing me right to the house.
A second negative, which I hadn't considered, is that I can easily see and read the Garmin on the dash without taking my eyes off the road. Although the in dash unit is at least twice as large (screen), it's location requires a much longer look to see what is going on. This has nothing to do with one brand vs. the other but has convinced me that a top of the dash unit is preferred to a lower mounted in dash unit.
The cost of the updates is a negative for the Garmin, but other than that I am very happy with it.