Hi,
Is SatNav a good thing to have? Yes.
Is TomTom better than Garmin or vice versa? That's very much up to you - map data, turn point and positional precision are identical - the real difference is in the user interface and which you find easier to use. You need to try them out before you buy.
Both are good quality makes with lots of faithful followers - I have a TomTom 700 with full maps of Europe on a micro hard drive and my wife Brenda, and her 70+ year old parents, have the Garmin i3 models with UK mapping.
Neither TomTom nor Garmin have let any of us down, and the bargain prices currently on offer for the Garmin i3 make it very attractive - but only if you are happy with UK only mapping.
If you do need France or Benelux, please consider spending a bit more on an upgradeable Garmin or any of the TomTom models.
Actually you can buy additional European maps for the i3, but the i3 transflash memory card is not meant for repeated removal and re-insertion, and you would be better served by the SD memory cards in the more expensive Garmin and TomTom units.
And it's a good idea to have a paper map in the boot - as both a back-up and if you want an overview of a route from say Oxford to Porthmadog, you may want some idea of places and towns to avoid or include on your SatNav route.
Robert