Hi Chris,
I first purchased a Garmin unit shortly after they were released. It served me well until it was out of date and updating was about $100.
I bought a Tom Tom top of the range a year ago. With life time free updates.
Also for SWMBO a Garmin. With free updates.
Differences:
New to old Garmin; much faster log in. Easier to use. touch screen.
Tom Tom. I now prefer this one. It is registered to be used in 45 countries.
It does not drop off the windscreen in heat like the Garmin does.
Bigger screen
Has live road conditions, free for first year after that a fee. It is fairly accurate for arrival timings. Suggests alternate routes before hitting the road. Very useful in congestion situations.
Both units are excellent. Within 5M accuracy. Garmin is slightly more user friendly but that may be because I was initiated on the Garmin.
However as I mentioned my choice is the Tom Tom but it is double the price of the Garmin because of all the countries for which you get life long free updates, bigger screen and a host of choices of voices colours etc.
Cheers