Is it better to go harder/shorter or easier/longer at the gym?

I'm trying to get healthier and I noticed that when I do the elliptical trainer I need to stop after 15 minutes but I can sit on the stationary bike for an hour. Looking at the calorie count it's about even (if the machines estimates are correct) so which is better for me? To exercise harder for a short period of time or lighter for a long period of time?

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