This is a statistical fallacy. Two blocks will always be equally difficult when they were mined with the same target.
This is a definitional fallacy.
What you mean to say is that clients prefer to choose the block with the least block hash.
What you mean is the most difficulty, which is not the same as the numerical block hash. The natural numbers less than 2^256 are a total order, but difficulty is a partial order on block hashes.
For any target, blocks with these two hashes both meet that target. Therefore two blocks with these two hashes are of equal difficulty even though the first one has the numerically least block hash:
0xffffffffffffffffffff0000
0x000000000000000000000000000f0000