I sold my Zumo 550 for a Zumo 660. Overall I probably liked the 550 better. The 660 is 100% touch screen. No physical buttons to change screens. Unlike the 550 that allows you to hit one button to return to the map, if you find yourself 4 screens away from the map you will need to go back through all 4 to see the map again. The 660 rearranges the screens so you need to do less page to page changes, and more things are consolidated on a single page. Such as the compass being on the same page as all the details of the trip (max speed, distance, time, ...). This consolidation makes everything smaller which makes them harder to read when you're over 45 years old and needing reading glasses. My main motivation to purchase the 660 was to get Speed Limit data. Unfortunately I've found that most of the roads I ride do not have this data. The speed limit coverage is very spotty. Also the limitation on 20 routes has been a pain at time which has required me to unload a few routes to add another. The 550 allow more routes to be saved. Another shock was the missing trip meter. This may have been taken care of in a recent update, but I'm not positive. Until now all it's had is a mileage meter for since the unit was new, but not one that is resettable for a long multi leg trip. (again, I may be out of date on this fact).
The 660 will sense if it is in a car cradle and as you if you want to switch to car mode. That's all fine, but it will not sense that it's back in your bike cradle and offer to switch you to bike. So you need to navigate to the tools page to set it back.
The benefits are more memory in general, so downloaded map updates actually fit in the unit. The unit has a bigger screen but is thinner than the 550. Less expensive extra batteries. Plus it is the up to date unit and more likely to be supported by Garmin longer than the 550. Some say it is faster than the 550, but I don't really notice it.
Knowing what I know now I do not think I would make the same decision. I would have kept me 550 and waited for Garmin's next generation after the 660. Fortunately I was able to sell the 550 for more than expected so the change did not cost me that much. But I miss the buttons of the 550.
Don't know anything about the TomTom. Good luck.