Good question.
There is no absolute correct answer, as both options have their pros and cons. At the end of the day, you need to bring in as many users, on both sides (experts and people using their services) in order to create traction and gain revenues.
So, if the "Celebrities" are willing to endorse and advertise your platform, then it is better focusing on them, but in most cases, they won't do so, therefore, it would probably be better, and easier, focusing on getting regular (but quality) experts. Also, take into account that "celebrities" usually have less availability and are more expensive, which will affect the level of service that you offer your users.
Good luck
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