I've used both brands that you mentioned and I chose Magellan between the two.
The Garmin that I used were the StreetPilot II and the StreetPilot III. The difference between the two were size and the III had a touchscreen and used a regular compact flash card so you could save all the map info.
The two problems that I had with the Garmin is that you were limited to how much information you could have based on the size of the card you were using. I believe the card that they give you is a 256meg. I live in New Jersey and I was able to put in all the info for a trip to Florida. The info I'm talking about are points of interest, rest areas, gas stations, hotels, etc. The second problem I had with the Garmin was that it wasn't very accurate. I tested it around here and it would get me lost. I put in my friends address and it got me completely lost. It's supposed to recalculate the route if you take a wrong turn but this thing was very stubborn. I don't even want to get into the trip to Florida. I was going to visit a friend that moved down there. I punched in his address and it gave me the route. I drove around for an hour until I realized that I was lost. Turns out that he only lived about 20 minutes from where I was staying. Total piece of crap. Maybe they improved the system with the new models. Now, onto the Magellan.
I have the Magellan 700. Has a built in hard drive which means no card. Everything you need and want is on the unit itself. It also has a touchscreen. This thing is amazing. I've been to Florida, Maine, Vermont, and even Canada and it got me there. I haven't had any problems with the Magellan at all. I bought mine for 1300.00. I've seen them at Costco for 700.00 something. It's the way to go. don't mess with the card thing because you're going to limit yourself to what information you can have during your trip and you don't want to run into the situation where you need a phone number or address for a hotel and you can't because it's not loaded into the card.
Hope this helps.
Check out this site. They sell gps units pretty cheap.....
www.tigergps.com