51 percent attack is possible especially if the network hash rate is relatively small and thus will be easier for someone with enough resources and malicious intent to orchestrate such attack. You only need to control 51 percent of the total hashrate. That will be very difficult for for bigger network like that of bitcoin and maybe ethereum. This is one of the reasons why PoS is being proposed.where your control is based on the quantity of coin you own,someone who owns so much coins will not want to destroy the integrity of the network as he will end up being the one being hurt by such attack.
I don't think PoS could do better than PoW in stopping such a malicious activity. This is because if a party owns more than 50 percent of the total quantity of the coin in circulation then it is possible that such party could dictate and manipulate what goes on in the network. This may compromise the confidence trust of the network.
