Don't do decision maths! I got an A in every other module in A level maths and further maths but I got 61 in D1 because it was so different to the rest of maths. Even if it sounds more interesting, I know a lot of people who got worse grades in decision maths, and your grade at AS (so when you've not had time to resit) will be important for medicine. I'd recommend doing statistics just on the basis that you might get a better grade.
Universities won't care which maths modules you pick because at a lot of school students don't get to choose their modules, so I wouldn't bother choosing a module based on how prestigious or relevant it is to medicine.