Have a Garmin Nuvi250W that I bought in Feb this year. A bit disappointed with it. A lot of recent new roads, bridges, by-passes etc, some as old as three years are not mapped. Also it can take a good 5min to get a Sat signal after turning it on. Another thing I found with it was that when I was driving in Vienna, with a lot of high buildings around, it lost the signal, so back to the old hard copy map........
No option on the Garmin website to alert them of mistakes on the map, such as new bridge in Navan over the Boyne that's open 3 years but a no show on Garmin, also by-pass of Roosky, Co. Leitrim, again a no show on Garmin.
Have no experience of TomTom, but my sister in England says her one has a lot more features than my Garmin.
Only advise I can offer is to ask friends if you can borrow their GPS for a road test to see which is most suitable for you.