Canada is a pretty big country. What part(s) are you interested in?
In general, if your travel will be mainly in, and between, major population centres, Garmin's City Navigator North America 2009 (NT or non-NT, depending on your nav device) would be the best choice. That includes regions like southern Ontario, lower mainland BC, Edmonton-Calgary corridor and cities of some significance like Winnipeg, Halifax, Regina, Saskatoon, etc.
Garmin uses data from Navteq for the City Navigator North America product. Navteq has little interest in areas of lower population, especially in Canada, but it does a decent job of keeping the more populated areas up to date ... keeping in mind that nobody's map data is much better than three years old by the time it is checked, packaged and hits the market as a product.
If you are interested in more rural areas, Metroguide Canada is the better product. It's much better with small towns and villages, and streets and roads, out in the boonies. I use it in western Canada in rural Saskatchewan, Manitoba and Alberta and the interior of BC. It also has far superior representation of geographic features like parks, lakes, rivers, etc. The geographic features are complete and accurately represented.
For Metroguide Canada, Garmin used data from DMTI Spatial, a Canadian company with superior map data for all parts of Canada. They also supplied the data for Garmin's Topo Canada maps. I have both and it looks like the geographic features in Metroguide Canada are lifted straight out of the Topo Canada data. If you turn off the elevation lines in Topo Canada it is impossible to see any difference in the geographic features between the two products until you get zoomed in far enough that you can start to see all the small creeks and little trails and fire roads and that sort of detail that you only get in Topo Canada. In fact when you get into the Rockies and zoom way in it's surprising how many of the forestry roads show up in Metroguide Canada.
Unfortunately, Garmin has decided to abandon the Metroguide Canada product at version 4. They still sell it but they have decided not to do any further updates. So if you need up to date urban information you will be disappointed.
I use both City Navigator North America and Metroguide Canada. I update City Navigator about every second version so my city information will be reasonably current. But I still use Metroguide Canada more often because I do a lot of travel in rural Sask and Alberta and in the Rockies.
Your best bet is to go to Garmin's site and look at both products using
Garmin's online map viewer. Look at the areas that interest you in both products (you can open up two tabs or windows in your browser and open one product in each) and make up your own mind.
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