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?