Ok, I'm not currently at uni at all, so I don't have 'insider information'. If that makes you stop reading what I have to say, fine.
Firstly, what year are you in? Just so I can get a better idea of how early 'early days' really is.
Secondly, these courses are very different, probably more different than you think. Medicine is a vocational course, and as such there is relatively little flexibility in the academic modules. There is also the three years of clinical training, followed by two foundation years and (I think) up to six years of training in a specialism. So, you've really got to be in it for the long haul, willing to be puked on at stupid o'clock in the morning by a drunk in A&E. In order to even get to medical school, you will need a load of experience: not only with a GP or in a hospital, but also volunteering with elderly people/children/people with disabilities to prove you have the personal skills. A medicine PS needs to be 100% dedicated to medicine and cover more than just the science.
Natural Sciences, on the other hand, is about as broad as a degree program can get. It's good if you're strong in more than one area of science and/or want to do something interdisciplinary. While they will expect your PS to show some idea of a preferred area in science, they will also want enthusiasm and for two or more different (though likely related) areas. As it is not vocational they do not care so much for work experience, though they will want to hear a bit about what you do outside school.
In a way I find it difficult to understand why you are conflicted between these degrees, as one is extremely specialist and the other very broad. However, I did almost this last year. From year 10 I had been doing work experience in preparation for a vet med application. But by year 12 my heart was telling me my interests did not lie with this specialist degree and I should go for a regular science degree. My head on the other hand, told me to go through with it - after all I had spent so long preparing and I could still go into research after a vet degree. The scale of my mistake hit me like a freight train in the middle of my Cambridge interview after I'd spent 20 minutes talking about my passion for research, when they asked me why I hadn't applied for Nat Sci and I couldn't provide a sensible answer. I ended up with one offer, though not from Cambridge, which I turned down and am now reapplying. While my grades are not good enough for Nat Sci this year, I am much happier with my course choices than I was last year.
Bottom line is this: Any medical course is a huge long term commitment to a very specialist career. If you're questioning whether you want that, it's probably not a good idea to put yourself through it. The med schools may also pick up on it at interview, so you may find yourself with nowhere to go and an unplanned gap year.
However, if you're very young then forget everything I just said and come back to it before you start your actual UCAS application. A bit (or a lot) of indecision is perfectly normal when you've just started thinking about it, so don't rule out either course right away if that's the case.