I have a Nuvi 500 and a Nuvi 550. Suposedly identical except that the 500 has a Topo map and 48 states. The 550 has Canada and Alaska but no topo. However, the Garmin Topo is not very good, I deleted it and installed a better free topo from
www.gpsfiledepot.com. So in that sense I think the 550 is a better buy. That is why I bought one when I thought I had fried my 500.
But the two units actually have other differences as far as I can tell. Windows reports the 500's internal memory is about 1 GB larger than the 550. It also seems to me that the 500 has a more "twisties rider" friendly recalculation logic and is faster to redraw. But both are far better than my old Street Pilot. I prefer the 500 but glad I bought the 550 in case I ever get to Canada or Alaska.
One drawback they both have is the lack of an audio output jack. Instead there is a small built in speaker that can not be heard about 30 mph. Somebody on some forum has wrired in a jack but I am afraid to try that.