In that case, my advice would be to go with hindi. It would be easier to learn if you already know a bit of the script because its a phonetic language. In addition, I don't know which part of India you're going to, but since its punjabi, i'l assume north india, you will find many more people speaking hindi than punjabi. Especially if you're going to Delhi.
Hindi at an advanced level is pretty hard, at least I found it so, and i lived there for 17 years
, but just basic speaking/listening, should be quite easy once you know the script. Since you already know quite a bit of it, that's a head start anyway in learning it.
Punjabi is pretty similar to hindi, but if your dad speaks both, I would advise going for hindi.
Hindi at an advanced level is pretty hard, at least I found it so, and i lived there for 17 years

Punjabi is pretty similar to hindi, but if your dad speaks both, I would advise going for hindi.