Today i recorded the same route from both a garmin edge 305 and using strava on my nexus 6p. The Garmin appears to win. The Garmin on on a flybar mount on front of my bike and the phone was in a ziplock bag  in my back jersey pocket, so it might be interesting to do some further testing with it in a similar position to the garmin. The garmin data is fed to garmin connect, which is in turn fed into strava.

Nexus 5P and Strava



Garmin 305

The garmin output appears to be less jaggy and follows the road or the path more accurately

I also have an ANT+ usb adaptor on the phone and the heart rate monitor was emmm poor

Nexus 6p

Garmin Edge 305

I had thought about switching to just using my phone, but the garmin is giving me consistently  better gps mapping.