Yup unfortunately they aren't cheap. In terms of HR and Pace I did quite a few tests - in the beginning I also wasn't completely convinced so had to prove to myself it worked. I have a hill here in Krugersdorp which I aptly named "The Nipple" as it has a nice symmetrical shape ( )and I ran a few tests based on Pace, HR and then Power. I set my Garmin to beep when I went over or under the required value - and ran them all on the same day of the week at the same time - for consistency. With HR I found that about halfway up the hill my HR spiked, so I slowed down to keep in the required zone. It then remained high until over the hill and only came down again halfway down the other side - so I was able to speed up. When I ran on power, the moment I hit the incline my power went up instantaneously, so i slowed down, but as soon as I reached the peak, my power dropped and I had to speed up. When I downloaded and compared, I was 6sec faster over the segment than when I ran on HR, and more importantly, my HR curve was a lot more consistent than when I did the run on HR alone. So in summary, I ran the hill faster with Power, and at a constant effort, compared to when I ran it on HR alone. It seemed to me that my HR lagged over the hill - so I was losing the potential to increase my pace in the downhill while I waited for my HR to recover. That was the deciding factor for me. On Pace it was a bit more blurred - the total time on pace vs power were very similar, except my HR was very high on the uphill and very low on the downhill running on pace, whereas on Power my HR was more consistent over the hill - due to that fact that my pace dropped on the uphill but sped up on the downhill. So running on pace I guess over shorter distances would probably be ok, but the continuous spiking in HR over a half-marathon and longer would probably not be ideal.
