Well, the Instance based Power report gives you the power consumed by all instances that are present in your design. For example, if your design instances some IP or some compiled blocks such as adders,etc, these are called instances.
Cell power gives you the power taken up by each cell in your design. Cells are the standard cells... (I may be wrong here)!
Instance based power will give you power of each individual instance in the design, where as cell based power will sum the power of every instance of a particular cell.