You've hit on two of the four biggest differences - the other two being that the 550 speaks the street names for the upcoming turns and comes with the automotive mount and power cable (as well as an AC adapter, but many never use that anyway).
The car cradle and power cable will cost you about $100 if you want to use it in the car without pulling the cradle off your bike.
You can use the M C mount between a M C and car and do not need a second mount.
I used the MC cradle, had a suction cup mount, made a power cord to plug into the cigarette lighter and bought a cassette adapter to get the MP3 and GPS prompts through the truck speakers. If you don't have a cassette player in the vehicle you could use a FM modulator. Note the inline connector on the power supply. Makes for a quick swap between bike and truck. On the bike