This is my humble opinion but as far as I understand it, the Proof-of-Stake model offers better protection from 51% attack as the attacker would have to own 51% of all coins which would be virtually impossible to accomplish (even if, say, the Chinese Gov't wants to buy all ppc the price of the ppc would increase exponentially, making it impossible even for them to own the majority). - It's easier to control majority of processing power than majority of coins. - This is just my partial understanding of the PPC whitepaper, I'm still struggling with it.