When I first got interested about the viva someone told me, “People hopefully pick an internal who they like, and hopefully pick an external that is an expert in their field.” I’m not sure that’s the best advice for picking examiners, but it’s a starting point for today.
It could certainly help to have an expert for an examiner: then you would feel confident that they would understand the implications of what you have done without needing to spell it out again.
But suppose they know it all too well? Suppose they understand it better than you? Suppose they can appreciate problems that you haven’t? Suppose, suppose, suppose…
Don’t forget: by the time you submit your thesis you are an expert. No-one else has done what you’ve done. No-one else has your lived-in experience of doing your PhD. Maybe it takes an expert to examine an expert?