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
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
Garmin Edge 305
I had thought about switching to just using my phone, but the garmin is giving me consistently better gps mapping.