I don't know much about Magellan's products or their service, but I know from first hand experience that Garmin's service is fantastic. I am very happy with my older model Garmin (GPS V) that I have had for many years now. I use it heavily for off trail hiking and hence use topos. I believe it is true with units from either manufacturer that unless you have topos for some specific, specialized area (such as the topos for the National Parks) they do not have great resolution. At least for the Garmin maps, you can look at them online (
www.garmin.com) and see if they meet your needs.
If you need more info, I would recommend the collection of reviews at
http://gpsinformation.net/ For input from actual users of each of these models, you might try the "gps units and software" section of the forums on geocaching.com,
http://forums.groundspeak.com/GC/index.php?showforum=11
Good luck. I have been thinking about upgrading for sometime. I will definitely get a Garmin since I already have a fortune invested in Garmin software, but I haven't convinced myself that the new features are worth the money compared to staying with what I have at the moment. I will be anxious to know what you decide to get and how you like it.