The advantage of using a strict prompt -- one that either dictates the exact words used in the opening, or one that demands to be used in a specific way -- is that you can systematically compare the different ways your students used the prompt. Interpretable prompts don't allow for the same type of comparison, but because they allow students to interpret them and connect with the material on a more intimate level, the writing tends to be stronger and more emotionally charged.
Using dialogue as a prompt can stimulate interesting and exciting writing, but it has the drawback of forcing the students to begin their pieces with dialogue -- unless you specify that the line can serve simply as a stimulus for the conflict. For example, the line: "For the last time, I told you to start your homework," could easily begin a written conversation, but it could also serve as the catalyst for the conflict; it could be what has taken place just prior to the first lines of the story.
Sometimes the best prompts are those that are both open-ended and stir strong feelings. Introspective phrases are usually highly interpretable and can often be used initially as an opening, but then be removed afterward with no negative consequence to the writing. For example, the prompt, "They just don't understand" can serve as an opening phrase. If the whole sentence is "They just don't understand that I will never forgive him," the prompt can be removed and still leave the strong opening: "I will never forgive him."
A tone-setting prompt is one that orients the attitude of the writer to a certain extreme, which will color the voice and style of the writing. Tone-setting prompts can function as opening phrases or simply as internal stimulus. For example, the prompt, "How dare you..." could be as an opening or simply as emotional firepower for a piece of writing.