51% attacks against Proof of Stake blockchains are hard to attempt. You can't just rent a large amount of hash for a few hours. You literally need to control a lot of coins to do this, more than 50% of the total staked amount. You could purchase coins, which increases their price as you buy them. You could steal them, which would be such a big theft it would get noticed. There'd be less incentive to kill the chain with a 51% attack than it would be to just sell the stolen coins. These are some of the reasons Qtum chose Proof of Stake. The cost of such an attack would probably cost tens of millions of dollars.