I don’t think the answer is one or the other.
I think the answer lies within the spectrum and depends on the kind of task you’re hiring someone for.
If you want the world’s expect on a certain kind of brain surgery, there are just a handful of those folk out there. And, if all of them have a bad attitude, there’s not much you can do beyond not offering brain surgery as an option in your hospital. Expertise matters.
That said, there are many many jobs that don’t require much expertise. They can be done by anyone above a basic threshold of capability. And, yet, we see job descriptions that list a long laundry list of basic/preferred qualifications. I would imagine they would be better served if they just said – “Need someone with some grit and a very positive attitude.”
I have found myself kicking myself every time I’ve looked for expertise. I’ve almost never been disappointed when I’ve just looked for attitude.
Maybe the way to think about it is – if you really really need expertise, go for it. If not, once folks are above a basic expertise level, default to hiring for attitude.
It just works better that way.