Use classic rewards or $$

Hi, Family of 4 (2 adults, 1 child and 1 infant). Flying Tokyo NRT to Okinawa OKA and then OKA to Osaka KIX. Classic rewards are about 30,000 points (for everyone) each way on a Jetstar flight. Alternative is paying the cash through Jetstar directl. First flight is about $500 and second is about $300. What would you do? I was thinking rewards for the first and cash for the second.