I liked the Garmin for its simplicity, and it freed up the smartphone for my wife. The Garmin also has images of the freeway exits and the lanes you needed to be in. This itself paid for itself when travelling, especially with a trailer. We were able to travel through LA pulling a trailer without a lot of stress. One wrong exit and you can be in big doo doo!
A data plan with Google maps is better for traffic, so my wife would often peek at Google maps and I would then make a decision to detour if I felt it was necessary.