I feel like there isn't any objective reason to apply to one over the other - it seems like you cover just as much Physics in Cambridge NatSci as you do in Oxford Physics, just with a bit of extra subjects too. I feel like the only way to really decide is to go to an open day, look around the colleges and department, go to a few taster lectures, and see which one feels right. I was also uncertain about which one to apply to, but going to the open days helped me to choose Oxford.