I don't really know which is better, I have seen Lenovo AC adapters for the US market with a C7 cable which were double insulated, the equivalent European part was not double insulated and had a C5 cable (thus also using grounding).
I assume that if Lenovo is okay with shipping AC adapters without grounding but with double insulation, that they are essentially equivalent (probably with some cons/pros for each solution).
Obviously the third pin on the C5 is ground, so I am guessing it does not really matter too much, as long as you don't use a non double insulated adapter with a C5 plug without proper grounding (that said it will probably work just fine anyway - but at your own risk of course
).