In the last 2 years I have used for collecting data various device combinations (Garmin 520 with wahoo speed sensor and wahoo cadence sensor, Garmin 520 with wahoo cadence sensor only, Garmin 820 with wahoo cadence sensor, Garmin 820 with Garmin Vector 2 for cadence & power, iPhone 6s with wahoo cadence sensor on strava - strava does not use data from a speed sensor but only the gps' & glonass ones - iPhone 7 with wahoo cadence sensor). Lately I only use Garmin 820 with Garmin Vector 2. Two main reasons, battery life of Garmin vs iPhone and tons of data information collected through Cycling Dynamics. Out of my little experience I can say that on road cycling the accuracy of a gps unit vs speed sensor is almost identical.