[nadam]

"do you think it's more important to have a deep knowledge of a few topics or a fair understanding of a lot of topics"

I have been thinking a lot about this. My conclusion is that the optimal thing is somehow both. When I say both, it sounds bullshit, but it is not. I think the key is to be ABLE to study specific topics deeply, know what is deep understanding, know what really tight focus is, learn and practice one or two specific topics very deeply AND with this kind of developed mind, go into lots and lots of quite diverse topics following your interests and needs. And be brave to sometimes dig a bit deep into something which no one expected you to dig into.

So optimally your knowledge should be 'incalculable' by other people. A kind of interesting, a little bit strange knowledge-portfolio which is not very common.

TL;DR: Go very deep into something (extremely deep if possible), learn a little a bit of everything (extremely only the basics about extremely lots of things) to avoid knowledge holes, and go a little bit deep into unexpected things to make your knowledge-base incalculabe and interesting.