Have extensive experience now with (older) Garmin Etrex 20 and many mobile phone apps. There's a good chance I'm out of date but key points for me (probably in a random order) are...
Unless you want to do something fancy like measure heart rate the simple advantages of a dedicated GPS unit are no longer accuracy, but mainly robustness/waterproofness and battery life.
In my experience mobile GPS accuracy caught up with dedicated (consumer level) devices some time ago (have tested in difficult conditions).
The Garmin survives rain and a battering on the handlebars that the mobile wouldn't.
The Garmin - as said by Cyclops - will run for a very long time on AA batteries, which are easily replaced/recharged.
Consider whether you actually want to do more complex navigation on the Garmin, or to take a mobile. I now ALWAYS do the thinking/planning/decision making using my mobile, while the handlebar mounted Garmin provides the ongoing map in front of my eyes, and tracking my route.
Make sure and understand the capacity for all of these devices to use Openstreetmap data. This is what I use for all but the most adventurous walking/cycling. The only times I use something else is if I need the rich detail provided by the OS 1:25000 for very rural off-road across-the-hillside trips.
There's no issue at all, even on my old devices, with having the whole of the UK Openstreetmap data on both devices. Actually there's little issue with also having several other countries on there too. I imagine we're getting rapidly closer to the point where we can carry data for the whole planet around with us...
The Garmin mapping styles seem more limited than what I can see on a mobile. I'd want to research what Openstreetmap mapping can look like on comparable devices. On the mobile there are hundreds of alternative attractive/useful map styles. Do any of the Garmin units allow this flexibility yet or are the displays still a bit limited?
And what I particularly like is that I have two devices so backup if one battery causes problems.
So - like I say, this may all now be out of date, but seemed worth adding...