Chain with most proof of work - hash target or block header hash?
It's the sum of difficulty targets, not the individual difficulty scores. Therefore, two blocks at the same blockchain height are always the same cumulative difficulty¹.
If it were individual difficulty scores, it would make chaintips more gameable, e.g. a block with a particularly low hash might prompt a selfish mining attack because the attacker could expect that their chaintip will outdo another even when they reveal the block later, or miners might continue mining at the same height because an already discovered block barely met the difficulty statement. In other words, the blockchain would converge less reliably if it was based on individual block scores.

¹ Unless we're in a longer blockchain fork across a difficulty reset which changed the difficulty by different amounts for the two chaintips.