Not so much a brand vs. brand thing, I'm telling you from experience. Garmin will be a lot more accurate, has support for more datums, comes with a better base layer for data, support for consumer use, etc. Magellan uses mostly support via plug and play SD cards, cool idea but try taking that outdoors under conditions, report back. There is a reason the Govt. uses Garmin over Magellan, accuracy on a good day rivals that of Trimble, don't believe me PM me.
External Antennas are a thing of the past, you do not need a large bulky antenna to get good reception, and Garmin proved that with its quad helix back in I think 2003.
Support for base layers will be found more readily available from Garmin as they are based out of the US (Kansas I think), unlike Magellan who is out of Tales(sp.?), France. Base maps are designed in a propietary format, with less limitations on Garmins design, giving you more options to upload third party base maps such as tools found in google earth or a more common sled trail layer you can pull from some states.
Not trying to sound like an azz to vmaxfrank, Magellan has its place, and it is mostly in aircraft because they can lock the prgrams and the recievers down to perform specific duties.
I am just a firm believer that as a consumer product the Garmin series handhelds are designed more btter to reaching their target markets.
