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.
This would be a special case situation in which no one is using PPC for currency and in which we disregard any effect of PoW I would think, as PPC only generate stake blocks after sitting and doing nothing for 30 days. In the case where 50% of the coins in the network are being actively transferred between persons, you only need 25% or more of the network to perform such an attack using stake blocks if we were to disregard PoW entirely. The more coins that are being actively traded, the less stake required to perform an attack.
But PoW is never disregarded by the chain, rather Sunny King just assumes that PoW blocks will be mined with exponentially increasing hash rate with the same overall power output. PoS actually makes it quite a lot easier to attack the network in the event that the PoW hash rate isn't very high and a lot of PPC is being actively traded, so long as the attack is well premeditated (as you need to wait to accumulate coinage).