Garmin

The 770 is slightly larger in size (both body and screen). The 770 is a higher resolution screen though Garmin's graphics don't really take advantage of that. The 660 comes with a traffic receiver in the power cord that receives the over-the-air traffic signal in major cities. The 770 does not come with this traffic receiver. Instead, it requires the use of the Smartphone Link app to data-share from your smartphone over Bluetooth to receive traffic. This traffic service is more robust than over-the-air traffic but with the 660 you have the choice.

I've read limited reviews that the 770 is having problems with BaseMap.

The 770, being new, should get you a year or two more support in terms of firmware updates before being discontinued.