I use both: GPS via phone app, and older style Wahoo magnet type speed/cadence sensors paired with the bike computer. There's hardly ever much difference between the two readouts on segments or overall rides.
I also have one of those no-magnet convertible earth induction speed/cadence sensors but haven't used it for awhile. Results were comparable to GPS and magnet sensors, but with more dropouts and glitches.
I don't worry much about speed. Mostly I use the computer to monitor my heart rate, distance and ride time.
The cadence info was useful last year when I switched my style from spinning 90-100 rpm to slower 60-75 rpm. But now that the slower cadence is in muscle memory I don't really need the reminder anymore.