On PoW, if one fork is much less popular, it will be feasible to attack because you can rent enough hash power to produce a larger chain. If only slightly less popular, this isn’t feasible.
On PoS, a substantially less popular fork will be possible to attack, because you can afford to buy enough coin to stake and take control of the validators. The economic value of your coins will be divided between the forks based on market consensus. A more popular fork will be hard to attack because the coin will be too expensive.
In my view these are a similar type of security guarantee, what am I missing?
In both PoW and PoS, providing security to prevent attacks requires allocation of economic resources (spending hash power, or holding and staking tokens).
In PoW my GPU can only hash one chain at a time so I have to pick a fork to continue on. This is true for everyone else too, so the incentive is to mine the fork that will most likely live since you can’t earn coin without mining.
In PoS, I hold tokens and when a fork happens, I’d hold tokens in both forks now (same parent token in both child forks). I can trivially run 2 validators where I stake my tokens in both forks. This is possible because validation in PoS was meant to not consume much resources, so running 2 is affordable. The incentive is to stake both forks because doing so means I’ll pick the right fork (by picking all) and thus preserve my stake in at least one fork.
I think the punishment system described elsewhere in this thread solves the issue of existing validators running both chains. If that’s correct you’d have to buy more coins to stake to “attack” the cheaper chain (which is much like renting hash power to attack a PoW chain).
On PoS, a substantially less popular fork will be possible to attack, because you can afford to buy enough coin to stake and take control of the validators. The economic value of your coins will be divided between the forks based on market consensus. A more popular fork will be hard to attack because the coin will be too expensive.
In my view these are a similar type of security guarantee, what am I missing?
In both PoW and PoS, providing security to prevent attacks requires allocation of economic resources (spending hash power, or holding and staking tokens).