Both platforms are strong, well supported and functional. However when it comes to both

• variety of interoperable/compatible hardware
• number of areas, throughout the world, covered by available maps

Garmin is superior. In my case, I needed a navigator that can be used both in the whole of Europe and in Brazil. TomTom up until recently did not provide maps of Brazil that could be added to a TomTom unit, although there were TomTom devices being sold in Brazil that have a first version of TomTom Brazil maps pre-installed. The Brazil maps have very recently become available but there are still some that countries/continents that Garmin covers and TomTom does not. On the other hand, TomTom may have an edge when it comes to detail and coverage of Europe, although it's hard to tell as the existing maps of both platforms are in constant update.

Overall, if you wish to invest in a navigation platform that is truly global and includes a very wide variety of compatible devices, I would highly recommend Garmin.