Metaphor and simile
1. A figure of speech in which a word or phrase that ordinarily designates one thing is used to designate another, thus making an implicit comparison.
2. a figure of speech in which an expression is used to refer to something that it does not literally denote in order to suggest a similarity.
3. One thing conceived as representing another; a symbol.
1. figure of speech in which two essentially unlike things are compared, often in a phrase introduced by like or as.
2. a figure of speech that expresses a resemblance between things of different kinds (usually formed with `like' or `as')
As you see, metaphor and simile are similar. They serve like purposes. Metaphors however are a purer form of symbolism, and leave your reader to make an inference as to the subject of you poem, rather than making it obvious, as does a simile. Metaphors deliver your message with more emotional impact than direct language, or similes. However, be careful not to use clichéd metaphors (Life is a roller coaster, etc) or you nullify the effect. Also, when using metaphor in a poem, use the same metaphor throughout. Be consistent and avoid mixed metaphors. To mix metaphors also weakens the effect and weakens the piece by confusing the reader, particularly if the metaphors you choose are vastly different.