Doesn't a fork in both cases continue with the existing chain? In PoW that means that mining power has to be allocated between the two chains, thus constituting a vote for legitimacy. In PoS each party keeps their stake in both new chains and can vote in both of them. There is no need to pick a lane.
The validators choose which to support. If you think one will fail, you will be selling off the forked eth, bridging it over, and buying more eth on the chain you want to support
