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.
They put on a magnificent match where it was truly hard to say which was better.
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 Japanese rendering of the Chinese idiom 難兄難弟, originally applied to two brothers whose merits were so similar that neither could readily be ranked above the other.