both are equally excellent; hard to choose between two beautiful things
Idiomatic expression comparing two things that are both outstanding, making it difficult to decide. Often used when both options are so attractive that either choice is fine.
Both of these designs are so beautiful that I can't choose between them.
Both her proposal and his proposal are equally excellent.
Similar meaning of being hard to compare, but 甲乙つけがたい is more about difficulty in ranking or judging superiority, while いずれ菖蒲か杜若 emphasizes equal beauty or excellence.
Standard kanji form of the idiom.
Variant using the kanji 何れ for いずれ.
The idiom comes from the difficulty of distinguishing between the iris (菖蒲, ayame) and the rabbit-ear iris (杜若, kakitsubata), both of which are beautiful purple flowers that look very similar.