Ahh the big Garmin, TomTom debate....
TBH there isn't much in them... TomTom has its advantages and Garmin has its own advantages.
In your basic Garmin satnav you get, full postcode lookup, street name, house number (takes your right to the door)... Points of Interest near you or in a city.... Speed camera locations (fixed camera) and thats basically it (Garmin are a more basic satnav)
In the basic TomTom range, you get all the above and you get extra functions such as diversion (if you find a roadblock you can re route yourself for 1-3 miles around the block, great for motorway accident traffic) you also get to use the TomTom as a road atlas scrolling around the country selecting roads and things like that.
TomTom release updates pretty much once every couple of months, which will update the firmware on the unit. There is also more for the TomTom as to voices and add-ons then there is for Garmin.
TomTom do take a lot more to maintain though... Need to keep on-top of the updates, and make sure you make a backup of the WHOLE satnav as soon as you take it out of the box, as sometimes the TomTom can crash (nothing a plug into the computer and a restore can sort out)
Garmin don't release as many updates, and you are stuck with the one voice.
I'm nit picking here, as they are both great units! I get to play around with both of them all day every day, and i couldn't decide between the two (which is why i have a TomTom and a Garmin

- Only because of one of my websites)
Hope that helps you a little