Too Long; Didn't Read
There are three key reasons why PoS is a superior blockchain security mechanism compared to PoW.
PoS Offers More Security for the Same Cost
The easiest way to see this is to put proof of stake and proof of work side by side, and look at how much it costs to attack a network per $1 per day in block rewards.
GPU-Based Proof of Work
You can rent GPUs cheaply, so the cost of attacking the network is simply the cost of renting enough GPU power to outrun the existing miners.
For every $1 of block rewards, the existing miners should be spending close to $1 in costs (if they're spending more, miners will drop out due to being unprofitable, if they're spending less, new miners can join in and take high profits).