To put it quite simply:
Grammatical choice has to do with differentiating between two or more verb tenses (past, present, future, past-participle, present-participle, past-progressive, present-progressive, future-progressive, perfect tense, imperfect tense) and determining which verb tense/form is correct under the given literary circumstances within a sentence or paragraph.
Rhetorical effect refers to arranging words in a certain order or context to create a more poetic effect with their literary arrangement thereof. Poetry bends the rules of English grammar quite often in order to design a more artistic, rhythmic display of language. However, the meanings/definitions of the words used poetically with rhetorical effect remain the same contextually.
I hope this helps, Shella! Feel free to let me know if you have any questions.