I would have to pick negative. To my mind, I am an adult and most rewards are things I can get for myself regardless of whether or not I accomplish the task. For example, I've tried to force myself to sit and work for one full hour with the reward being that on the hour, I can get a snack or go have a smoke. Well, that didn't work because I can get up any time I please, nothing to stop me. So a consequence would work better for me and only if someone else is in control of it.