Etrex 30 has compass and barometric altimeter, plus can connect to ANT+ sensors for heart rate, cadence or temperature.
I don't think the compass or altimeter are very useful for cycling. And they can be annoying or inaccurate at times, it seems there is no way of disabling them.
Plus the Etrex 30 can connect to other Garmins, to wirelessly transfer waypoints/routes etc. And it can be used as a remote control for the Garmin Virb camera.
I think the main thing is do you care about measuring heart rate, or cadence or temperature. If so get the Etrex 30, plus you'll have to buy the compatible sensors.
If not, just get the Etrex 20, its a fair bit cheaper.