IMO the 500/550 is tougher.
The 550 LOOKS tougher with its chunky design but both 550 and 660 share the same plastics and have similar touch screens. They both have the same spec for water resistance. I have read worries about the battery/SD card cover on the 660 but have not had any problems and there is no reason to think its much different to the hinged flap on the bottom of the 550. There is no practical reason to believe one is tougher than the other.
Having had a 550 and a 660 (plus a Garmin 12, 2720, GPSMAP 60csx and Garmin mobile on a Palm and sadly a Tomtom Rider) I can say that 550 or 660 will do the job (never had a 220).My wife now uses my 550 and has been very happy with it (she also has a Nuvi in her car).
-The 660 is a bit easier on the pocket when its not on the bike.
- Its easier to get to the SD card on the 550 but that's maybe a once a year job anyway.
- Both have had rare lock ups on the road requiring a battery removal to reset. That is a sod on the 550 but takes a few seconds on the 660.
- The extra screen real estate on the 660 is nice but in practice you navigate from bottom to top so the extra width is no big deal and in any case part is used for soft buttons which replace the hard buttons on the 550.
- The Bluetooth on the 660 is a different flavour. Some phones work on one and not the other so if phone compatibility is important this is worth checking.
- If all of Europe is needed the 660 has a much bigger memory. Until the latest update the whole of Europe fitted on the 660 but only about half goes on the 550 with the rest having to go on an SD card. Not a deal breaker as there is the sd card work around.
- The 550 cradle mounting looks more secure but the security screw is a nightmare (a thumb screw replacement is possible) and the two pins on the mount can jam in the holes in the satnav. Although the 660 mount looks a bit precarious it is quick to use and I have never had a problem and have not seen any reports of it failing.
- The car mount for the 660 is much simpler as the 660 has a built in speaker and mic.
- There are a few more bells and whistles in the 660 software but the only one that I use is the more detailed map screen.