Is there an idiom for when two things are the same, like tomayto-tomahto, that works well in written form?

It's six of one and half a dozen of the other works in both written and spoken English without modification. Wiktionary:

(idiomatic) The two alternatives are equivalent or indifferent; it doesn't matter which one we choose.

It's just a matter of semantics.

From an earlier thread: [If we consider the way in which the word 'semantics' must be interpreted in the fixed expression 'It's just a matter of / merely semantics']

• ... ['Semantics' in this sense may be seen] as being about very fine distinctions, such as: I think it's just a matter of semantics, not so much a difference per se.

• Another interpretation seems to be that 'semantics' is the difference between two ways of saying the same thing.

Essentially, just a situation (often an argument) where the terminology, the precise meaning of words used by different parties, is different, when essentially the same thing is being said in different ways. Examples include:

• But this is largely an argument of semantics. [Wired]

• Some of that is simple semantics, but other concerns are harder to dismiss. [The Verge]

• In other words, this vaunted farewell is more or less a story of semantics. [Slate Magazine]

[Cambridge Dictionary]

The expression seems too transparent to consider it a non-analysable idiom, not venturing to define the sense of 'semantics' involved.

Another idiom that could be used is two peas in a pod. Lexico has:

So similar as to be indistinguishable or nearly so.
‘they were two peas in a pod, both with the same high cheekbones and hairline’