I’ll prefix this by saying that as a medic I have absolutely no knowledge of either NatSci at Cambridge or Earth sciences at Oxford, but that I do have first hand experience of applying to Cambridge. The application process for Oxbridge is incredibly tough and I would warn you that certain components can look quite straightforward, whilst actually requiring a lot of time and effort to prepare for (if it’s difficult you’re going up against some of the best students in the world in a difficult test and if it’s straightforward, you’re going up against some of the best students in the world in a straightforward test. Doesn’t make much difference if you catch my drift). The thing that does make a huge difference with Oxbridge is your passion for what you want to study, this is one of the key things they look for (once you’ve satisfied the academic requirements) and if you feel that Environmental Sciences at Oxford is more centred around the topics you’re passionate about then you will do better should you interview for that subject.
At the end of the day, both are amazing universities and both are ridiculously competitive, so the key thing that sets you out amongst the field of other (incredibly well qualified) applicants is your passion. This is one of the few situations where as a scientist you’re better to go with your heart and not try to crunch the numbers in your head. It’ll be better for you in the long run to heavily research both courses, see which one teaches the most content that you’re passionate about and then pick that course.
Best of luck.