I have a Garmin 505, which is excellent for me - but i have often wondered about our relative outputs. We bought a cheap (€30) watch style heart rate monitor (incl strap) from Decathlon and my wife attached it to the bars - works a treat and lets us both have a conversation about the big thing - are we both working equally hard! - which can easily be gauged via heart rate. Works well in long alpine climbs where keeping a consistent heart rate is key - and gives the stoker something to ask you about for a change.