I understand what you are asking, because I've wondered the same thing myself. I had a Nuvi 660(?) for a few months and am now using a 680(?) - I'm guessing on the model numbers, basically I had the traffic.com one previously and now I have the MSN one.

It *seems* so far that the traffic.com had much wider coverage and seemed to know about roads that were off the beaten path and not just in the urban areas.

The MSN one seems to have *better* coverage in the areas that it does have coverage, ie: it says if you are coming up on an accident, construction, etc, with a little more detail.

It's still too early to tell and form an opinion, but I'm in the same boat.

The MSN service does have a gas price listing, it downloads the gas data every few hours, the data is somewhat mixed - it tells you the age of the data, most of it is within 3 days, but it only seems to list (at least here) gas stations that take credit cards, as I think that the listing comes from visa card transaction records centrally. It also has movie listings and weather, which is nice.

The traffic/gas/weather receiver is built into the cigarette-lighter adapter, which means that if you are using the Nuvi on battery only, you don't get any traffic info. The traffic receiver is supposed to be able to keep receiving data for several days even after unplugged. When flying into other cities and plugging it in, it definitely takes several hours before it is "caught up" with the local information, then it is usually fine for the rest of the trip. Because it can only go a couple of days without power, you really need to keep it plugged in all the time. A friend used to keep it in his glove box and just plug it in when he was going to use the nav and always complained that the traffic info was never available, which makes sense.

I'd love to hear if anyone else has opinions on the services (I'm in california) - I know you can go to traffic.com to get coverage information, not sure about the other one.