A and B; first and second
Used when labeling or comparing two items, options, or parties, often in formal or legal contexts. Common in phrases like 甲乙つけがたい (hard to choose between the two).
It is hard to say which of the two is better.
The contract requires signatures from both Party A and Party B.
difference in quality; superiority or inferiority
Refers to a discernible gap in quality, rank, or merit between two things. Often used in negative constructions to say there is no significant difference.
There is no difference in quality between these two proposals.
If there is a difference in performance, I'll choose the better one.
It's hard to say which of the two is better in skill.
There is no real difference in quality between these two products.