Lithium-Ion (18650 cells) versus Lipo -- which is best?

If you fly an RC plane or drone you have typically used a lithium polymer (lipo) battery but there is an alternative -- the lithium ion cell, typically in the 18650 form-factor. What is the difference? When would you use Li-ion in preference to Lipo and vice-versa.
I will shortly post some practical "field test" videos showing how great (or small) the differences are.