I have a Forerunner 305 and an Edge 800.
For cycling, the edge is better – simply for the bigger screen and ability to display maps. It can also be mounted on the quick release wrist strap for the Forerunner 310XT, so COULD be used for running if you don’t mind a bigger device on your wrist.
If you run, the Forerunner 310 XT might be a good compromise if you don’t need maps. You can select bike or run (or other) and customise the fields displayed for each. it will show cadence with the GSC 10 sensor, and speed also becomes very accurate with this. It’s auto-calibrating using the GPS, so a doddle to set up (at least that’s how it worked on the Edge).
Garmin Connect is brilliant. Very easy to upload data to and analyse post race.
If you don’t want the expense of those, the Forerunner 305 is still available for just over £100. very versatile, does most of what the 310 does.