First off, good for you on gaining the experience in other fields! Being multi-faceted only makes you more employable.
Second - and take this with as many salt grains as you like, since your question is asking anecdotally - I'd go with scenic. I find, again in my personal experience, the overlap there is more prevalent and useful when stage managing.
Now why not props you ask, when most small-time jobs might lump together SMing and properties acquisition? Because it's more specialized and the knowledge base isn't as large as scenic skills (again, anecdotally, please don't yell at me for dissing props masters or something). A SM who can judge structural safety competently and professionally, or add to the conversation of how to accomplish something technically is more valuable.
Have I mentioned this is anecdotal?
Ultimately though, choose what makes you happy! What are you most curious about? Do you have any previous experience with any of these that could influence you?
And really: does the assistant design position actually do any of the work, or are they there just to shadow? Choose the one with the person who is willing to give you the best learning experience.
Also, anecdotal.