I have a TomTom One XL-S in the car..works very well. When I tried them out, the TomTom just seemed to be more intuitive when inputting addresses.
Items that Garmin might do better: I think it tells you what side of the street your destination is on...which I hadn't noticed until I was in a friends car...very handy. Also, on the TomTom map version I have, it seems to use the pre-Toronto amalgamation name as the Town name. Thus, I have to know if a street is in Toronto, or North York, or Etobicoke..which is a little annoying at times.