Strictly speaking, these questions have different meanings and occasions in which should be used (usage patterns). In practice people often use them interchangeably, and the meaning is inferred from the context. English is a living language and therefore it is dynamic - the "rules" sometimes done reflect actual use... but they are the rules.
I'm using examples from the Education First (EF) English grammar web pages.
The precise usage patterns are:
Will you be visiting your aunt while you are in Paris?
When discussing a plan to be in Paris at a known future time; does the plan include visiting your aunt (staying with her for some time)?
The question uses future continuous tense/aspect, which is used for several purposes; The EF page includes the statement: "The future continuous refers to an unfinished action or event that will be in progress at a time later than now". But it also mentions that "In the interrogative form, the future continuous can be used to ask politely for information about the future".
Are you visiting your aunt while you are in Paris?
This sentence is in Present continuous. The usual interpretation would be a discussion about the present. It could be either
- You are currently in Paris and the question is about whether you are also currently visiting your aunt
- You are going to Paris regularly (e.g. every year in June, starting in the past and continuing into the future) and the question is about whether you have a habit of visiting your aunt when you are there.
As stated in EF, present continuous can also "describe an action or event in the future, which has already been planned or prepared". But this is usually done by adding some time description like "next week". In this case there is no hint that it discusses the future, so it is not a likely interpretation. For discussing a future plan, use "will you be visiting" or "are you going to visit".
-- edit - additional information on future simple --
I was asked in the comment about using future simple "Will you visit...". The "Simple future tense" EF page states that
With you in the interrogative form, to give an invitation: Will you come to the dance with me?
Will you marry me?
However. I'd say this is a very strict prescription; people use "will you" in interrogation (questions) in many other cases too, and the meaning is often clear from the context. If you were to ask me "Will you visit your aunt while you are in Paris" I would understand it as a question about my plan.
If you want to be really precise in your expressions, you may choose to avoid "will you" in this case.
Note that the "Future" EF page nicely summarizes the attitude attached to each of the forms used for discussing the future.