With PoW, if a node is sent two copies of the blockchain, it is trivial for the node to determine which version is valid because it can trivially calculate which blockchain has the greater amount of total work.
I thought PoS also adhered to the "longest chain" principle?
Usually,
PoS is the Longest Chain with the Highest Difficulty.
Difficulty is determined in some cases by Number of Staking coins * Age of coins since last staked
PoS implementations are incredibility versatile, so different coins may have other variables that determine difficulty factoring in pool variables.