The 78 series is a major upgrade over the 76Cx.
A basic comparison is at http://www.gpscity.com/compare/76csx&78sc . A good review is at http://gpstracklog.com/2012/08/garmin-gpsmap-78-review.html
1) Key pads are about the same. You won't notice much difference.
2) User interface is different (an improvement IMO). While many screens are almost the same, how you access them is a bit different. Although the 78 is not touchscreen the interface now incorporates many features from the Oregon (touchscreen) interface. You will need to spend some time learning the new layout. I was very frustrated for a few days due to the differences but now prefer the new layout.
3) I have never setup a route on a GPS, I use software like Homeport for that. A great thing about the 78 series is that you can have 200 routes with 200 legs each. The 76 has only 50 routes with 50 legs. What that means, for say, the Everglades Challenge, is that you can now have a route go from one checkpoint, all the way to the next, with plenty of legs (the disadvantage is that if you are using your old 76 as a spare, it will truncate all the routes after the first 50 legs).
The 78 series gets a lock quicker than the 76.
For fitness paddling, if you get an ANT coded strap, you can view your heart rate on the 78.
There have been several times where the newer interface has been very irritating, with me failing to find what I'm looking for while on the water (and requiring a Google search later on), but I can say that for every model GPS I have used.
Pull-Up Strap Handle Kit
Paddler's Truck Rack
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