Hi there! I use a Polar HRM to track my workouts (I only wear it when I'm working out). I usually find that my HRM trends HIGHER on the calorie side, but because of that I usually deduct 20% of the calories it gives me. I don't feel that either way is more accurate than the other, but I really like how the HRM takes into consideration my age, weight, gender and how hard I'm working (using the monitor).
When I compared my Endomundo application vs my HRM, I found the HRM to be higher.
I don't know that there's an exact answer here. :)