We have had the Etrex, the Map 76, and then the map 76CX and each one has been an upgrade. We purchased the 78's a few weeks ago, thinking that they would be an up grade, and they turned out to be a down grade.
My comparison will be between the 76Cx and the 78: The 78 uses batteries quicker. The tide chart is ridiculous. You find a tide station that you want, and you hit the "go" and it wants to navigate you to that tide station. When you finally do get the tide chart, there are no cross hairs that give you the tide position where you are, vs, the nice accurate ones on the 76Cx. The graphics are not nearly as clear and accurate, and you can see other screens in a dark shade in the background and the individual screens are smaller. The north point is smaller. You need a magnifying glass to see the scale. When you plug into a computer, it only acts as a USB mass storage unit and can't be used unless unplugged from the computer.
We found that to get to a particular screen, you usually had to an extra step than we did with the 76CX Lastly, the micro SD card is located in a delicate hinged metal cover under the batteries, and when I went to change the battery, both the cover and the card flipped out. I was at a table, but if I were in my kayak they would have been long gone overboard.
The 78 might be good for land use or geocaching, but it is definitely a step backwards vs the 76Cx. Unfortunately, Garmin is not making the 76Cx any more...