Looking at the specifications on Garmins website for the nüvi 760
(link) and the nüvi 680
(link) ,which is a discontinued product, the differences appear to be that the 760 offers:
nüvi 760 includes advanced navigational features to take the worry out of traveling. With route planning you can save 10 routes, specify via points and preview simulated turns on the 760's wide screen. In addition, nüvi 760 automatically sorts multiple destinations to provide an efficient route for errands, deliveries or sales calls. It also displays speed limits for highways and Interstates. A trip log provides an electronic bread crumb trail of up to 10,000 points, so you can see where you've been on the map.
Auto sort multiple destinations (provides most direct route):
Where Am I? (find closest hospitals, police & gas stations, nearest address & intersection):
Garmin Locate™ (marks position when removed from windshield mount):
Qwerty or ABC keyboard (choose keyboard layout):Edit to add - Here's a side-by-side comparison
(link)