When should we use -ing or infinitive verbs?

I am in doubt about what is the correct form of the verbs remember and understand in the options to the first question below:

Which is more difficult in learning a second language?

1. Remembering vocabulary or understanding native speakers? (Gerund)

2. To remember vocabulary or to understand native speakers? (To-infinitive)

3. Remember vocabulary or understand native speakers? (Bare infinitive)

Remembering vocabulary or understanding native speakers? (Gerund) is significantly better than either (2) or (3).

A fourth option is possible: To remember vocabulary or understand native speakers? (Mixed)