Loan Payment Question

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I am new to the forum so please excuse my inexperience. I am an 18 year soon-to-be college student and I am in contract to purchase a coop in NYC. I took out a 30 yr fixed for $265,000 and my parents are helping me out with the $131,000 down payment provided that I pay them back within 15 years with 2% interest. My interest rate for my mortgage will be somewhere between 4.5 and 4.75%. In planning out the next few years of my finances I am trying to determine where to put my spare change. On one hand, it seems more beneficial to make additional principal payments on my mortgage because the interest rate is higher. On the other hand, The window of time I have to pay back my parents for the down payment is only half as long. Do I make additional payments on my mortgage or on the loan from my parents? Thanks!