PoW works nicely in p2p settings, from small clusters to Internet scale. However, at Internet scale over a long period of time not clear sustainable model.
PoS not suited for small p2p settings below some monetary threshold t. At Internet scale threshold t becomes easily surpassed, just need to ensure a relatively “fair” distribution.
Peercoin does PoS consensus with PoW distribution.
2nd layer solution could be PoW consensus (PrimeCoin 1 minute blocks) with periodic PoS blocks as anchors for proof of ordering.
If your short-range consensus is PoW, then over a long period of time the unsustainability will generate the possibility for double spends and network manipulation. For example, a political movement that has >50% miners collude against certain holders of the coin.
If your long-range consensus is PoS, then you run the risk of someone forging a similar chain in private (for example, getting control of the stake modifier) and causing a deep reorg.
Having PoS as the main consensus mechanism with PoW injecting entropy as Peercoin already is is a fairly well-balanced approach.
What about atomic swap between Primecoin and Peercoin, similar to Bitcoin and Litecoin? Technically what makes Litecoin superior to Primecoin? Litecoin just has better marketing and more active developers.