HR Monitor VS Runtastic and Elliptical Calorie Read Out

nicknpick

I just got a HR monitor, and used it for the first time today. After using Runtastic to keep track of my walk, and the calorie read out on my elliptical I had a calorie burn of 840. My HR monitor said over 1000. Which should I go with? Is there a way to test the accuracy of either? I know nothing will be 100% accurate. I just like to know if the HR monitor is worth keeping!