I’ve owned a couple of Garmin models. My experience is that Google Maps does a better job of finding the most efficient route. This isn’t always true, but when Garmin is wrong, it’s very wrong (I’m talking miles out of the way for no apparent reason).
I was very upset when I bought a brand new model (nüvi 2455LMT in 2013) and the internal memory wasn’t enough to update the maps without adding an SD card. (You had ONE JOB, Garmin!)
Garmin does a better job in areas with poor-to-no cell reception. Yes, Google Maps “works” offline, but my experience with that is hit or miss, especially if you want to enter a new destination while offline (e.g., I had to drive back to my hotel to get onto the wifi to search for a restaurant).
Garmin also tells you the speed limit, which is very nice if you are spending much time on country highways where the limit could be anywhere between 50 and 65 mph, and the signs are few and far between. And, if you don’t trust your vehicle’s speedometer, Garmin tells you your currents speed.
Garmin is better if you travel where cell access is limited, if you want to be able to use your cell for other things in the car (e.g., you let your kid watch videos in the car), or if you want the added info it offers like speed limits.
Google Maps is generally better at efficient routing, includes information about things like public transit & walking routes, and is much less expensive if you already have a smart phone.
Oh, and one other note, depending on the laws where you live, using a maps app on a smart phone may be a legal issue. I’ve been told that it’s potentially problematic where I live (that you aren’t allowed to touch your phone while the car is in gear), and that the same standard doesn’t apply to a standalone GPS units.