To attempt to answer your question, there is no real difference in map coverage between the Garmin Units and Magellan, to name two. Both have their mapping software created by the same company, DMTI Spatial however to proprietize the software, it was developed to be incompatible with each other. In other words, Garmin mapping software will only work on a Garmin and Magellan software will only work on a Magellan. Lowrance is another GPS manufacturer, however they developed their own software, rather than had it producted by another company. It then boils down to who has the best firmware/hardware.
My personnal choice was Garmin, hands down. Most commercial and private aircraft use Garmin equipment for navigation, which is what sold me on that brand in the first place. Pilots depend on accurate guidence systems to bring them to their destinations, which Garmin has perfected. I have never been disappointed with my choice. I did noticed a lack of detail when it comes to the mapping software for Ontario, that it was slightly less detailed than Alberta. You have to remember this is an American company who maximizes their efforts to please the American customer. A LOT of Americans go to Alberta and BC to hunt and fish.... no so much to Ontario. This possibly explains the detail varience between Ontario and the western Provinces.
I hope this helps you with your choice of handheld Unit. No matter what brand you buy, here are a few links to get you the best deals out there:
GPS Central located in Calgary, Alberta usually has the best prices on any make GPS, and they are a Canadian company.
http://www.gpscentral.ca/http://www.gpsworld.com/http://www.garmin.com/garmin/cms/site/ushttp://www.lowrance.com/http://www.magellangps.com/Cheers. Dave