Any ANT+ sensor will work.

If you install a power meter, it will probably send cadence data (mine does. It is a Stages). The advantage of using Garmin's cadence sensor is that it takes longer to disconnect. My power meter will disconnect from my computer after about 2 minutes of inactivity. A long descent or waiting for others is enough to require a manual restart. The inconvenient of Garmin's is that it is easy to lose. It is attached to your crank by a gasket-like contraption that wears with time.

GPS-based real-time speed estimates are much less accurate than sensor-based. Doesn't matter for casual riding but could be annoying for serious training.