Why would competition from Ethereum change once they switch from PoW to PoS?
That is like saying PoS is the only feature of Peercoin.
When talking about the PoS algorithm. The biggest difference is that on Peercoin every coin is one vote.
While on Ethereum, the system is designed so that there is competition between minters, minters can gain advantage by gaming the system. This means that the process of finding stake becomes more expensive and this cost should be paid by the network, which yields higher inflation rates.
You could argue Peercoin has PoW which increases inflation rate, which is true, but it provides us entropy to ensure our naive PoS algorithm is secure.
Compared to Ethereum, Peercoin uses two simple and proven algorithms, while ethereum uses one complex and unproven algorithm.
But like I said, there is much more to peercoin than just the algorithm.