I’m not sure about Ethereum PoW, but in Bitcoin you absolutely pick the chain with the most work not the longest chain that’s a consensus rule. This is how bitcoin/PoW avoids weak subjectivity, the problem described by the parent comment.
The two are equivalent if each hash is valid. The only way for a longer chain to have less work is if there's a sudden, protocol violating drop in the difficulty in a block header. If this happens then you know that chain is not the valid chain.