I'm in the market for a new GPS, so I went to the Garmin website and compared the actual specs between the 770 and the 660 models.
The primary difference seems to be the size of the screen and the quality of the image. On the 660, the size is 6.6" wide by 3.7" high, compared to the 770 screen size of 6.8" wide by 3.9" high. While these differences don't seem like much, it extends the diagonal measurement from 6.1" to 6.95" and screen size from 24.42 sq inches to 26.52 sq inches, about 8.6% bigger.
Also, the screen resolution improves from 800 x 480 pixels to 1024 x 600 pixels, a noticeable improvement in viewing.
As far as the software, the units are very similar, as the website does state that both have voice activated navigation. Some 660 owners have stated they do not have voice controls, so it appears later 660 models have been upgraded.
The biggest operational difference I can determine is that the 770 has added integration to the travel service "Tripadvisor" so that it provides information on hotels, restaurants and other points of interest on screen while traveling.
Comparing prices on Amazon, the RV770-LMTS is available for $399.99, while the RV660-LMT is available for half that amount at $199.98, both with Prime shipping.
While these old eyes would welcome a bigger, better screen...it is hard to justify paying twice the price. Your opinion?