how does PoS choose a 'winning' chain I often see the 51% attack described by saying that if Bitcoin is presented with two conflicting chains, it will select the longest one.  This would seem to work because difficulty only re-adjusts at certain block intervals, meaning more hash power makes mining faster possible, until difficulty adjusts, at least.

On PoS type coins, difficulty re-adjusts every block, so even if you bring a bigger gun in terms of more coin or coin-age, you should theoretically, even on a shorter scale mint the same numberof blocks as the 'main chain'.  So if there is no 'longer chain', how doe PoS select a winning fork in the case that one occurs?
Usually longest chain is selected as main chain.The other condition is if two users find two blocks at same time.The block which is approved by maximum nodes in the network will be considered and will be added to longest chain.And the other block is considered as orphan block and considered as forked chain.