To make a decent recommendation, you need to specify how else you may want to use the GPS. Will you ever want to carry it in your pocket or use it on a boat?
For what you've said, I'd get one that has a 12V card cord plug, one that can take an external antenna (and buy a magnetic mount antenna to use on your vehicle), one that is backlit when using the 12V card cord, and most importantly one that can load street maps for your area.
I have a Garmin GPSMAP60CS which works well. But this is a hand held unit and intended to be used that way most of the time. I sometimes use it in the truck with a suction cup mount and it works well. For something that's only used in a vehicle, I'd get one with a bigger screen and meant to be used that way.
The TomTom is by far the better navigation system for in the vehicle. I personally have a HP Pocket PC with the TomTom software and GPS antenna, If you go that route, make sure you spend a little extra and get a bluetooth GPS antenna. The TomTom is pricey but worth the extra expense. http://www.tomtom.com
When it comes to the hardware, get anything with a SiRFstar III chipset. The brand isn't important. Anything with a SiRFstar III will smoke all other chipsets.
GPS is a little like computers that way. Intel and AMD engineers are the real developers of cutting edge technology, not Dell. The same goes for your GPS receiver's sensitivity. SiRF engineers are the real developers. Garmin provides the package and software. Those are important. But they become meaningless if the frickin receiver can't pick up a signal. Start with SiRFstar III and then worry about software.