Over at Groundspeak, Geocaching.com‘s message forum, Peter gave a great explanation of the differences between Garmin and Magellan topo maps:

"As Sputnik indicated, the approaches taken by Magellan and Garmin are different. Garmin uses Digital Raster Graphics (DRG) to basically create an image of the USGS 1:100,000 series maps. Therefore their maps have all the detail and accuracy provided by those maps, and nothing else – which means they lack most road names and anything built since the maps were last updated (20 years or so ago in many areas). But they do show the features included by the USGS: springs, fire roads, quite a few trails, etc. Also note that the 1:100,000 maps are metric so the contour spacings are based on rounded numbers of meters. When your GPS is set to feet the spacings look a bit strange (e.g. contours at 164′, 328′, 492′, etc.)

Magellan’s topo maps are instead based on the Digital Elevation Model (DEM) technology where elevations are sampled in a grid pattern and the contour lines are then recreated by interpolation between grid points. That provides the contour line detail but by itself wouldn’t give you anything else. So Magellan combines this topology data with the same road data they use for their street maps (based on TIGER for the original Streets and Topo programs, based on NavTeq for DirectRoute and 3DTopo).

If you want to get by on just a single set of maps then Magellan’s approach is clearly superior. But anyone who wants auto-routing and elevation data (both good things to have) will need to get both sets of maps anyway. In that case each approach has its good and bad sides. Magellan lets you see all the data at one time on your screen since their street data is identical on both and therefore doesn’t cause a problem, but they lack many of the older dirt roads and trails that are included on the old USGS maps used by Garmin. And, as Sputnik said, you can load both sets of maps in your Garmin and toggle between them. I frequently use CitySelect to get to a trailhead and then switch to Topo to see the terrain and trails for a hike."