Вот и про третьего вспомнили.

Now the advantage of a PoS system, is that it's naturally resistant against a 51% attack. To attack a PoS eco-system, you must have resources several times the value of the eco-system, otherwise, you WILL fail. On the other hand, to attack a PoW system, you just have to buy enough hardware to overwhelm the current miners, which currently cost about only 10% of Bitcoin eco-system value. This is why there has been exact ZERO successful 51% attack on any PoS alt coin, while plenty of PoW alt coin has been attacked to death.