For some reason, Option.fold is curried but Either.fold is not. All in all, I prefer the way Either does, it makes it so much more natural to use named arguments to disambiguate the call
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I'm not sure the inference ever helps in this case, anyway. But I suspect that the inconsistency comes from the fact Option is structurally asymmetrical, whereas Either was designed to be unbiased (but became such as an afterthought).
Option's signature is more conventional: it's easier to write a lambda as a single parameter in its own block than as one of several (consider multi-line blocks), but writing Either's the same way would bias type inference for one parameter return type over the other.