expression
both are equally excellent; hard to say which is better
Classical idiomatic expression used when two people are so outstanding that it is difficult to rank one above the other. Often used in literary or formal contexts.
彼らはまさに兄たり難く弟たり難しの名勝負を繰り広げた。
They put on a magnificent match where it was truly hard to say which was better.
この二人の作家は兄たり難く弟たり難しで、どちらが優れているとは言えない。
These two writers are both so superb that it's impossible to say which is superior.
互角 means 'equal' or 'evenly matched' and is common in everyday speech, whereas 兄たり難く弟たり難し is a formal, idiomatic expression emphasizing excellence.
A classical Japanese phrase derived from the kanbun reading of a Chinese expression. The exact origin is uncertain, but it is traditionally used to praise two people of comparable outstanding ability.