Be aware that the topo maps for both Garmin and Magellan are based on the USGS topo maps. These are out of date by 30 years, so the topo maps you get for your GPS unit WILL NOT show every road and trail that you might travel on.
Garmin also has a National Parks 24K topo map product (can't remember it's exact name) that is based on much more up-to-date information for the national parks and monuments. I believe some of the national forests on covered on that as well. I haven't purchased this particular product but people speak well of it.
I don't own a Megellan GPS, so I can't say whether they have the equivalent product to Garmin's 24K Topo map product.