Hi Duncan. I am in pretty same situation as you. I am looking for affordable power meter for my V20.
My favorite is Watteam PowerBeat.
I don't like pedal based PMs, I love my Speedplay pedals, but in this field I would prefer PowerTap P1 over Vectors.
With crank arm and spider based PMs I have a problem, because I am using Apex crankset shortened to 150 mm. I know that many cruzbikers use Stages PM.
My other option is PowerTap hub PM, but I it would be difficult for me to abandon my lovely DT swiss hub.
What I also like on PowerBeat is that it supports both Ant+ and BLE.