From what I know of medicine, you've already got your "main" subjects that are needed, so at this point it's mainly down to personal preference - just choose whatever you like more. Neither of which is too relevant to Medicine in particular, but they're both respected subjects, and in different ways provide some great skills. Would you consider taking either of them to Advanced Higher?
I can give some personal experience for Computing (I'm currently sitting the Advanced Higher, planning to go to uni for Maths and Computing). The Higher in many ways is quite similar to the Nat 5 in structure (the Adv Higher is a whole different beast). You've got the unit assessments (which are annoying but not too difficult), the assignment (a programming section, a databases section and a big write-up) and the exam.
I found that the Higher included quite a bit more on databases in the exams - getting a good understanding of them will be useful. You'll also need to know some more HTML and some CSS to write in the exam - this isn't too bad.
Apart from that, the biggest change is in how you explain things, both in your assignment and the exam. You've got to be a lot more thorough and precise with your vocabulary, using technical terms and all that. But this isn't unique to Computing in the slightest - it's like this for virtually all Highers.
In short, choose what you're more interested in - the main thing is that you can do well in it. If you'd like to know a bit more about Computing, just reply and I'll answer as soon as I can.