Cycling will burn off more calories than walking, providing you actually pedal the bike and not just let it coast down slopes, so try to find areas of flat ground or slighltly hilly areas, as cycling uphill will obviously burn more calories.
Bear in mind though, any activity that you do over and over, will just allow your body to adapt to that particular exercise, so it will just get used to it and perform less. Try to vary your exercise routine as much as possible, so it would be better if you do a combination of walking, brisk walking and cycling.