I have had a great deal of use with the zumo 660 and the 550, i have also used the TomTom rider series ( sell them at pony). I am a bigger fan of the versatility of the 550. The ability to do bluetooth stereo sound through head set (chatterbox xbi, midland bt2, or like) is present on both models for mp3 music, however the 550 does xm sat radio. The size of the screen on the 660 is larger which is attractive, but i believe would limit mounting locations because of size, for your bike i would suggest a tank bag, or more centered bar mount. The 660 uses a micro sd card vs a standard for the 550, might affect cost of larger units.
I would jump on a chance to buy a used 550 in a heart beak myself.
Review these features and decide for your self which one to get. https://buy.garmin.com/shop/compare.do?cID=135&compareProduct=15000&compareProduct=414
Both units are easy to use and rugged.Garmin's software is easily the best to use and upgrade, if the price of maps don't kill you.
If your intersted in a cheap alternative, the TomTom is still very very nice j at ust $450
PM if anyother questions i can help you with!