Writers are constantly drawing comparisons between apples and oranges. It’s an important part of our job – part of the delight of reading, for many readers, is seeing the similarities between two concepts that may appear to be completely unrelated. It’s clever. It’s interesting. It helps us maintain the illusion that there’s some order and purpose in the universe.
Many of us (myself included) think of “metaphor” as a catch-all term for comparisons made between unlike things. However, that’s not really true; we make a metaphor only when we make that comparison implicitly. “Your grandma’s a fox,” for example, is a proper metaphor. The reader knows from the context of common speech that the speaker really means “Your grandma is attractive and glamorous,” so no further clarification is needed.
That’s the thing about true metaphors: they lack explicit comparative signals, so you need some kind of interpretive context if you don’t want to lose the reader. If you don’t have the space for that context, then what you need is a simile. It does the same job, but it uses “like” or “as” to give you reader a clear signal that a comparison is being made. For example, “Your uncle is as mean as a snake” doesn’t leave the reader wondering how the uncle is like a snake; the comparison is drawn without excessive work on the reader’s part.
Whether you use a metaphor or a simile really does come down to how much effort you want the reader to put into understanding your comparison. This isn’t always an unpleasant effort – indeed, many of us enjoy reading poetry for the challenge of figuring out all the complex metaphors. However, if you don’t want the reader to be distracted by the comparison, or if you think the reader might be confused by it, then a simile is generally your best bet.