Translation:Is it better to read a lot or to see a lot?
At first I also thought "or" and "than" would be equivalent in this situation, but there are subtle differences. With "or" it is an open question, with "than" it is a closed yes-or-no question. Ths "than" variant would require ablative form for the direct comparison somewhere, I think.
'mi' is interesting because it doesn't have to come at the end of a sentence or clause... it can actually follow the specific word that we are questioning.
- Kahve mi istiyorsun? (I know you want something, but is it coffee?)
- Sen mi kahve istiyorsun? (I know someone wants coffee, is it you?)
When we are forming a "double-mi" question, each "mi/mı/mü/mu" will follow the two things we're asking about or comparing: in this case: okumak + görmek.
If it helps, you can think of it as keeping the sentence and options balanced. Here are some other Duo examples to compare:
Biyoloji midaha zor (yoksa)
kimya mı? [link]
evden migeliyor (yoksa)
okuldan mı? [link]
Yağmur muyağıyor (yoksa)
kar mı? [link]
Note: In a double-mi question, "yoksa" is optional :-)