I have the Garmin ECHOMAP UHD 93sv. At Bass Pro I asked the guy about it. It comes with Garmins LakeVU maps of pretty much most inland lakes, and the GT54 transducer. I asked him why one was on sale and the other was almost twice as much. The one on sale is about 5 years old, discontinued to be replaced with a newer unit, and thus marked down. The newer unit has the newest GT56 transducer and Navionics. I asked about the difference between LakeVu and Navionics. He said LakeVu was Garmin's own lake mapping but that Garmin had bought Navionics and added it to their new units. I asked if one mapping had a definite advantage over the other. His response was, "Nah, not really". I decided to save the money and get the older unit and figure it is adequate for my needs.
And I figure the best one can expect at Santee anyway, is using mapping to mark the established channels. Well, and like any other lake, channels, dropoffs, humps, or other underwater features in the deeper water.

I used my new Garmin on Greenwood this week, rather extensively just to learn the basics. First it is awesome. The only thing I need to work on is tweaking the color/contrast/brightness to see the objects on SideView. I see the shadows clearly but have a difficult time seeing what is casting the shadow.

The other thing I noticed specific to mapping. I was looking at a place on Greenwood where a side creek channel joins the main channel. The Navionics app on my phone, and the mapping in my unit showed the same bottom contour, however, they did not show me in the same position on the lake. Close, but not the same. Cannot say for sure which one was off but could have been one or the other was updating GPS signal at the time. I'll check that again to see if it's consistent or just happened at that point.