What burns more fat (biking wise)?

I ride my mountain bike everyday, but you might be talking about an exercise bike. Either way you should always mix it up by changing gears gradually. Staying on the same gear can cause a plateau effect from your muscles adjusting to the repeated stress. To burn fat you need to keep your heart rate up, start on the easier gears and gradually increase to harder gears after a few minutes, then come back down and use the easier gears as a cool down period before going up the gradual 'hill' again.