I have a 550 and have been using it for about 4 or 5 years now it was $899 when I initially made the purchase. It was one of the best purchases I made for the bike. The 660 has more internal memory to hold the new map which won't load completely in memory on the 550. The portion of the map that won't fit in memory I load on the SD card. I don't notice any difference in speed when it reads from memory or reads from the SD card. I also am using a Cardo unit, the first one I used was the Rider and now have the G4. Both units function very well with the 550. I contemplated the 660 and 665 but I didn't see anything there that would drag me away from the 550.
Let me know if you have any other questions about the 550,