Mountain biking vs road biking?

Hi. I love spinning at the gym and get my heart rate at 85% of max HR for an hour on average so its quite an intense workout. I want to buy a bicycle now but cannot decide between mountain biking or road biking. Which one would let you get your heart rate up the highest or which one is the most intense and also... show more Hi. I love spinning at the gym and get my heart rate at 85% of max HR for an hour on average so its quite an intense workout. I want to buy a bicycle now but cannot decide between mountain biking or road biking. Which one would let you get your heart rate up the highest or which one is the most intense and also FUN? Thanks
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