I am considering the 660 - not sure whether you could call it "seriously" though. Like the idea of the larger screen - 4.3" versus 3.5" for the 550. Don't really care that the 660 will not have the satellite radio.
Will look at what other features for both and make decision soon.
It looks like I will be looking at a 660. I currently have a Nuvi 660 and love the larger screen, and all the features. Waterproof , and being able to go on line and download routes are a plus for me too. I like the mp3, touch screen, and voice promps my Nuvi has. The only thing is the price. It's high.