Lithium Polymer batteries have many advantages over Lithium Ion batteries.
Li-Po batteries are lighter, they have a lower charging capacity loss, lower passive discharge and they can be produced in a wider of shapes and sizes.
But Li-Ion batteries have a higher energy density, so they can store more energy.
Lithiuim Ion batteries have a higher energy density per volume:
Watt-hours per meter square - Wh/m³, Watt-hours per liter - Wh/L
Lithium Polymer batteries have a higher energy density per mass:
Watt-hours per kilogram - Wh/Kg