The guys seem to feel the map system (vendor) is better on the Garmin units - - they use Navteq.
I've got a Magellan that's about 4 yrs old. Works great, but the interface is a bit too much effort. It's also slow when it comes to drawing screens and calculating. However, it has a very good PC based interface which the Garmin doesn't (available for $15).
I bought a nuvi360 about 2 months ago. I can't put the thing down. It's amazing and the user-interface is just excellent.
No personal experience with TomTom.
The prices on these things have just plummeted. When my nuvi was introduced a couple of years ago it was $900+. I paid $250 when I bought it 8 wks ago (via COSTCO). It's amazing what the $250 gets you. This thing is the entire AAA business model in one tiny package

The built-in database is vast, 6+ million points!