METs v HRM

I was wondering... when I exercise my HRM estimates I burn about 10-12 calories a minute. So a typical cardio workout for me would be 20 minutes running at 12km/h followed by 20 minutes of hill intervals - 2 minutes walking at 6.5km/h @ 6 or 7 incline followed by running up an incline of 5.5-6 at 12k/m for 90 seconds.

My HRM says I burn around 450 calories for this (then I subtract 55 because that's what I would burn at that time anyway according to my BMR). BUT, when I calculated the METs for this (using some chart on the internet) I got about 750 calories.

I know that BMR, HRMs and METs are all estimates so none of the figures are that reliable anyway but does anyone have any thoughts about which burn estimate to use? I'm entering maintenance now, so whilst before I just used to pick the HRMs estimate because it was lower (to be safe), now I'm not sure that's the best idea. Maybe I should just take the average?