It all depends on your navigation goals, I use both depending on purpose.
Getting from point A to point B in the most expeditious fashion? Nothing beats Gnav or Waze, so use the phone. Also, if you want to avoid the worst of the highways, make decent time, and don't want to plan a custom route, the Gnav with "avoid highways" is a good option that I've used to good effect.
So why do I also own a Garmin (really a BMW Nav5, which is a custom made BMW branded Garmin that integrates to the BMW mount and whiz wheel)? I like to do custom routes, as Gnav/Waze routes take you on I-roads, which IMHO is the most sucky way to travel by motorcycle. Trucks, buffeting, and lots of crazy car drivers who will kill each other to see who can arrive at their destination two minutes faster than the other guy.
I'm going on a trip at the end of the month, this is the Gnav route. I-75 and I-40 most of the way to the destination. Hot, boring, taxing, dangerous, and it bypasses all of the great twisty backroads along the way. But, its expeditious at 4:09.
But hey, I'm taking the whole day off for travel, I love twisties, I'm a map geek, so I put together this custom route, a route that in no way can be entered into Gnav or Waze. It's 8:57 in duration, but I get up and leave early and this route hits SO MANY of the great twisty routes in the region.
I have a "process" I use that I can share that takes a rubber banded route from Google Mymaps and eventually gets it into a gpx file that routes correctly in the Garmin GPS. It involves minimal use of Garmin Basecamp, which is good because Basecamp (sorry
@Ted , but it's the honest truth) is a horrible app with a horrible user interface that is perplexing to learn and use. BUT, it is needed to massage the route into a good gpx file.
I'll stop here, but if anyone is interested in the process I'll be happy to lay it out. But in the end,
@Morgan3820 , I use both phone and Garmin depending on purpose.