An even split fo far it would seem.
The issues as I see it go like this:
A pilot might lack the experience in industrial negotiations but would have the necessary insight. He would have credibility with the members and could be educated in the fundamentals of Trade Union law. However . . . What pilot would take this poisoned chalice on? Having put your head permanently above the parapet your chances of returning to flying might be compromised.
A non pilot could have industrial know how and previous experience of running an organisation. He might even come from another trade union if the deal was right. But would he really understand the pilots point of view? You could give him the insight through comprehensive briefings but operational experience would be difficult to acquire.
Who would be most credible?