How do you say two or several things are about equivalent to each other in English?
For example, if I post a question here and receive two answers which seem equally good to me, and I don't know which one to choose, could I say:
These two answers are equally good.
or
These two answers are as good as each other.
What's the idiomatic expression of it?
Best Answer
I'm afraid your two options are not equally good. :)
Although "as good as each other" makes grammatical sense, we rarely compare things like that. We usually say "equally X" to say that they're equivalent.
Intuitively, I think the two forms have different purposes.
When you say "X is as good as Y", the purpose is to use Y as a point of comparison to tell how good X is.
If this is the purpose, then it isn't helpful to say that two things are "as X as each other". Take "The sea is as black as night." The purpose is to use night to show how black the sea is. Night is your point of reference; you learn how black the sea is by considering how black night is.
So if I said, "The sea and night are as black as each other," we have a problem. "Each other" means that you learn how black the sea is by looking at the night... and you learn how black the night is by looking at the sea. That's a circular comparison, and not very helpful!
But the purpose of the word "equally" is the purpose you want.
"Equally" doesn't say how good two things are. It only tells you that they're equivalent.
It affirms that neither is faster or slower than the other — that there is no point choosing either one on the grounds of how good it is.
(You can also convey this message by saying that X is "no worse than" or "no better than" Y.)
Although Ngrams are not always useful, they can give us hints. Compare the rate of "equally good" vs. "as good as each other" in this Ngram. "Equally good" occurs 100-1,500x as often over various decades, which is such a high ratio that "as good as each other" is shown as a flat line by comparison.*
* Keep in mind that Ngrams are useful but not always definitive. See this discussion on ELU. It certainly doesn't mean there are zero occurrences of "as good as each other", and it also doesn't include similar wordings. For example, the classic children's book Onion John, which is full of idiomatic American English, says this in chapter 18: "Onion John could no more disappear out of Serenity, everyone agreed, than the courthouse could get up and wander off into the night. One was as much a fixed and regular part of town as the other."
Extra note
As a minor caveat, sometimes you will hear "as" used to mean "equally" to contrast with another comparative on the spur of the moment, and it will be stressed there. Here's an example:
If you want to say that she's not very friendly, you could say she's "only as friendly as Matthew" – but this makes Matthew the standard against which to measure Amy, consistent with the above.