The thing I like about the TomTom specifically is that they collect data from the user (from the actual GPS unit) review it for accuracy and then publish it hours later as an update. It's called "MapShare" and your TomTom can be updated nightly if you want to. TomTom also has a nightly update called GPS HotFix that helps the GPS unit locate the satellites and lock onto them.
You're able to make map corrections "on the fly" and they are instantly uploaded to TomTom, verified and offered back to the entire TomTom community - all within hours!!
Whichever unit you choose to go with, it's only as good as the latest updates offered to you or available from the company. You need to keep it updated at least once a month!
WoodGlue