Bill,
I say if you like your 550 and it's working well for you then stick with it. I like the 595 guidance directions that pop up showing which lane to be in, etc. but it's a quirky beast. I've almost gotten it figured out for my needs but still have to stop and restart the route so it will stop telling me to uturn.
I put all my routes and music on the SD card, importing and deleting routes as I go.