I usually just go with what SP says. In my case, my elliptical reads higher than SP. Usually my elliptical reads 425 for a half hour (I can input my weight, change the height/resistance..I usually pick a low speed and go 145-160 steps per minute...fast). SP says it is only 300 calories. I know that a heart monitor would solve the dispute, but I don't feel like spending the money. I'm only 10-15 lbs from goal weight, and workout for stress relief more than anything. I am in decent shape, so if I am dripping in sweat by the end, it was enough of a workout.
On the other hand, I have a cheap stairclimber and if I do 1 hour at the highest resistance (still super easy), SP says 600+ calories. But I barely break a sweat. So I know it isn't right.
I go by the sweat factor!