If you live in an area where GPS satellite acquisition is problematic, then an Edge 510, despite its issues might be a better choice than a 500. Yes the 500 has a better screen and is thinner and doesn't have as many firmware issues, but it is over 4 year old technology. The 510 has both GPS and GLONAS and acquires / holds satellites better than the 500. The Edge 510 isn't exactly cutting edge either as it is just over 2 years old. You can use a specific wheel calibration number with the 500 / 510 instead of auto calibration is there seems to be an issue with distance discrepancies.
Here is an complete review of the Edge 510:
Garmin Edge 510 In-Depth Review | DC Rainmaker
Even Strava Basic does more than record Speed & Distance, so it would benefit from the data collected by an Edge unit.
The advantages of an iPhone is that you usually always carry it with you and it is always on. Battery life is the biggest issue when using any smartphone as a replacement for a dedicated bicycle computer, more so as your rides get longer.