The last two guys mentioned hand held units that you can load maps onto. They are not designed for road navigation ie: turn by turn guidance. They are going to be much better for off highway use though, as you can pinpoint your exact location and go there (and also make various waypoints). You can even load topo maps onto the handhelds.
I own a Garmin 60CSX for my backpacking, and use a Magellan 3000T in my truck for road navigation. I would have opted for a Garmin C series, 340 specifically, but I won the Magellan from McDonalds, so that's what I use.
If you have money to spend, I would look into the Garmin C series units. They pack a lot of features, are going to be quick, and not too horribly overpriced.
My best recomendation would be go to a dealer and play with everything. Get the one you are most comforatable with, but know my vote is for a Garmin.
Oh, and a car navigation unit is goign to have a large touch screen, and the handheld unit is not going to have either (small non-touch screen) but they are going to be much more precise.