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I actually prefer Magellan, but probably just because I have used them a long time and like the interface. However, I would recommend whichever brand you get (and any is better than none), check out the line of Arkon mounts (Amazon has them). I use the one which mounts to the airvent on the left of the steering wheel. It is handy, and out of the sunlight so you can see the screen if needed. Probably you don't want larger than 3.5", however, as the space is tight, particularly if you plug in the power cable. To make the whole thing more stable, I put double sided foam tape on the "grabbers." Nothing moves, and I just leave it there all the time. The mounts are really inexpensive.
I have a Garmin 850 and have had very few problems with it. Since I've moved to AZ I find that the heat seems to affect it. So if I remember to take it off the mount and put it in my top pocket and then fire it up when I leave where ever I was it works better. If I leave it in the car in 110 heat outside and I have know idea how hot it is in the car it takes a long time to fire up. I bought the lifetime maps and so far they are as up to date as I need them.
I guess I prefer the Tom Tom but I have also used Garmin in the Vette, High-end units in both cases. I like the Tom Tom because it seems more precise in urban situations where lots of streets come together. I have one of the "Live" models. I made the mistake of thinking I could use the "pay as you go" model for the live services. Just try turning it off, you are stuck with a subscrition and they demand 30 days (another payment) to kill it. Get a Tom Tom but stay away from "Live" and get a smartphone.
I use a Garmin with the lifetime maps and traffic. I use a proclip mount for it in the Vette. My other cars I use a friction mount. So far the Garmin has been great.
I prefer the Garmins, I have tried the TomToms, but thet seem to have a lot of false directions. I fabricated a mount using a flip top ash tray insert and bolted the garmin mount onto it. I am able to set it into the cup holder next to the shifter. Works great!
I've had several Garmins and all but my last one worked very good. I just downloaded the Garmin App for my iPhone 4 and it is awesome, works better than my stand alone and has more features. It eliminates the need for an extra device and no cords.