in POS, you perform a complex calculation, but difficulties is much lower, while ANYONE can attempt mining in POW, POS, only people with coins in their wallet can attempt to mine when their turn is up. (coin maturity)
I dont know how Ethererum's POW would be implemented, but general logic should be similar.
Problem is.. POW benefits big hashing power owners (pools, miners with lot of rigs) POS, people with large sums of coins in their wallet wins over smaller wallet.
Neither POW, POS are perfect but each with their own benefits.