Which one(s) amongst the following sentences are correct, and which one sounds the best?
- "I do X if Y, and Z otherwise"
- "I do X if Y, or Z otherwise"
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To answer your question directly and literally as written, I suggest the second sentence sounds the best.
I do X if Y, or Z otherwise.
My reasoning is more based in logic than grammar. The choices are an "either/or" proposition (i.e., mutually exclusive) and not a "both/and" proposition.
Source: Native speaker