My mum is about 80 years old, and she uses a Garmin GPS unit. Her Garmin is quite easy to use.
While mum is not a technophobe, she is also not a technogeek, so having a reliable, easy-to-use system helps.
I have never used a tom tom GPS, but I have heard good things about them.
If you go to an electronics store maybe they could show you both systems and you could see which one you liked best.
NOTE: While GPS systems are quite reliable, if there have been significant road upgrades AND the local government has not updated their road infrastructure maps, then do NOT blindly follow the instructions.