Both use the same energy, in classical theory. However, the cyclist is required to do more work when pedaling uphill than downhill. Because friction is a non-conservative force that we must overcome to set an object in motion, we must constantly pedal on flat ground in order to propel ourselves on a bicycle. Even when going downhill, the force of friction must be overcome. If there is too much static friction (note that the type of friction between the bike tires and the ground is static because the tires are not moving relative to the ground) and it cannot be overcome, the bike would not roll downhill at all without some extra work being done to overcome friction. In classical mechanics, in an ideal world, the energy gained between the top and the bottom of the hill is always the same, regardless of whether you are cycling uphill or downhill. Uphill, though, you are required to work against both friction and gravity (gravity does negative work), so your muscles do more work on a microscopic level. Downhill, gravity does positive work and you don't have to work as hard to go down as a result.
