I think both of those items have to be on their own dedicated circuit because they are fixed in place appliances using over 50% of the circuits capacity.
That being said, if I was doing it I would put the light on the dishwasher circuit because I think the light would dim when the disposal turned on due to the small amount of torque in the beginning.
On the other hand however, the dishwasher is probably used a lot more than the disposal...